Actually, assuming that you use magic to make watery plants viable crops despite constant rain... you might find some information on fresh water aquatic crops useful.
Rice
Rice Fields - For fantasy purposes anyway, are those multi tiered things you see on hill and mountainsides. Water is channeled in from a natural waterway or reservoir at the top and carefully irrigated through everything.
Rice Paddies As Combat Terrain - Rice paddies are complex multi-teired semi-vertical mazes of tiny goat trails and shallow slushy muddy ponds with low walls (which could be soft clay or earth, or dry or even mortared stone or brick), the ponds could range from "dry" (but muddy) to knee deep, and may have rice in that could provide some grassy concealment late in season and no concealment at all when early in season. The major effects on combat is that you are going to be
over there and the place you want to be will be
up there and it won't be entirely clear how to get there, but it is entirely clear you are first going to have to slog your way out of this mud puddle you are stuck in.
Magic Rice - Real world rice needs carefully regulated irrigation and (fuzzy memory here) actually needs to be dried out as part of the final process to not just harvest it but to actual force it to produce actual rice heads at all. Magic rice in rainy land clearly just gets around that somehow and it's only demand is "moar water!"
Taro
Taro Fields - You may be familiar with Taro (or a close enough relative to get an idea of it's look) as "elephant's ears".
But go have a look.. An important thing to remember though is that
traditionally Taro was grown on relatively small scales. So for instance you could easily have your Rice Paddy village have what amounts to a couple of paddies dedicated to Taro instead.
Taro As Combat Terrain - Depth wise it's a lot like a rice paddy. Varying with crop phase (and Taro variety) from "dry" mud to shallow water. But the plants, are big, have trunks, potentially huge leaves (again depending on variety and crop phase), and could grow pretty thick (it's shady down in there). Also some forms will throw off runners from the base of the plant which will lace the field with natural trip wires that could easily be underwater, just at ankle grabbing height in the slippery or squelchy mud.
Taro As A Poison - Taro is a popular food crop in the real world from the Pacific islands to China. But there are a
lot of sorts of Taro almost all of them (barring some typical common name mix ups that aren't "real" Taro) are
Colocasia esculenta. This is interesting for several reasons.
1) That is a single species, it has something like
over four hundred varieties, many never flower, many in fact are
natural mutations rather than seedlings as many parent plants never flower but the species is especially prone to throwing mutant shoots. It takes a dedicated Taro specialist to tell the difference between many of these species.
2) This makes Taro an Aroid, and as such a member of a plant family that includes the SMALLEST flower in the world and the LARGEST one. Cool?
3) While various aroids are edible many are horribly horribly poisonous. The sort of poison that
immediately feels like hot burning needles in your mouth, then it gives you hideous stomach pains, damages your liver, sends you on a totally bad vision trip, and kills you! TARO IS NOT AN EXCEPTION. So you need to cook your Taro properly. You need to identify your Taro correctly (remember, over 400 mutants of the SAME damn plant, and you need a life time expert to be sure!). Even some edible forms can be rather poisonous unless properly treated and might even cause irritation if you get sap (or the freaky icky jelly on the exterior of the base of the plant's tuber) on you. And
even mild edible forms might be slightly "tingly" if eaten raw, and
are potentially linked with high rates of honest to god LEPROSY.
Magic Taro - Again you are making it rain resistant. But you might also make it GIANT! Because that is awesome. Also you probably have special herbal super poisonous magic Taros for poisoning fools and going on shaman voodoo trips with. Odds are good getting the raw untreated sap of those ones on you burns like acid then makes all your limbs fall off with instantaneous leprosy.
Water Ferns
Fern Fields - OK I don't know anyone who does this. However there IS
a certain water fern in Australia that was widely used by Aboriginals as a food plant. They just never grew it as a crop and harvested it wild. So it just grows around the edges of waterways, out of the water it looks like clover, only it never flowers and every single leaf is ALWAYS a "four leaf clover". In the water it floats it's clover leaves like lily pads. Now the thing is you COULD grow this stuff in mass. And as such would probably grow it in large shallow paddies or any available lake or body of water. I have even seen it grown in curtains through running water coming off spill ways and waterfalls. You could totally incorporate this right into a rice paddy structure, maybe even on the walls.
Nardoo as Combat Terrain - Nardoo grows in long, stringy, inter-entangled runners from which the leaf stalks uncurl in a spiral. It's very much trip over material, almost a net. You could totally pull up a sheet of the stuff as an almost instant camouflage blanket. Also those webs and nets not only run along from the bottom of a pond or down a spillway... they ALSO extend like a raft over deeper water, causing a potential for depth confusion and sudden plunges into water deeper than you expected. With a net on top of or entangled around you.
Nardoo as a Poison - The edible part of nardoo are its spore cases, which taste a bit like nuts. You can eat a few raw... if you feel game enough... but if you want to eat much you need to specially grind and possibly cook it. The famous white Australian explorers Burk and Wills famously learned just enough off the aboriginals trying to save their ill fated expedition form starving in the outback to know that the plant was edible... but decided to refuse further aid from the darkies, like say other foods and the way to properly treat the stuff. They learned the hard way that if you eat TOO MUCH of (pretty much any fern) and if you don't properly treat your nardoo... it causes a horrible vitamin deficiency, that kills you WITH BRAIN DAMAGE. This makes tribes that eat Nardoo strange and mysterious to outsiders because they are eating a plant that common sense tells an outsider is deadly, or that the outsider will foolishly go down the Burk and Wills path with...
Magic Nardoo - Actually this stuff totally has the best chance of any aquatic food crop of just growing in perpetual rain land WITHOUT magic modification. It is after all a fern and it CAN grow (not that great, but successfully) in actual permanent waterfalls. Where magic comes in is making OTHER ferns EDIBLE (many ferns will grow in pretty damn super ultra rainy conditions). Because if you can make the trunks of Tree ferns out of magic candy then your crop problems are looking a lot less problematic.
Sacred Lotus
Lotus fields - Actual disciplined lotus production fields come in two types, and both are
sort of boring. "Shallow" lotus is grown in about 0.5m to 1m deep water, deep water lotus is grown in something like 1m+ up to about 3m. At the bottom of all that is mud where the long tuberous rhizomes of the plant lie. Those are the main edible bit. But ALL THE REST is also edible (and otherwise useful) including shoots, stems, leaves, seeds and flowers. From that are the long (often slightly spiky) stems and the big round leaves and flowers, which rise up OUT of the water in large elaborate stacks.
Go Look a bit. . Of course if you want a COOL look. ANY body of water can be used to farm lotus, and in many real world countries ANY body of water IS used to farm lotus.
Lotus as Combat Terrain - Lotus is ALWAYS in the actual proper water. It NEVER has a "dry land" phase, and in fact would die if it were like that. It generally doesn't even have a "shallow" phase, it's full depth all the time. So your guys are all in deep water, possibly over their heads. When in season lotus is TALL so it's like being in a short but VERY leafy and thick forest, with scratchy stems, and all that knee/neck/deeper water, with mud underneath. Lotus LOVE nutrients, so farmers are likely to keep that water EXTRA super smelly. They may also run fish farming in there at the same time, probably giant magic mutant eels, I dunno. Lotus forms rafts out over deeper water JUST like the nardoo does, but on a MUCH large scale and with tall confusing leaves so you won't know you are in raft territory or not if you get lost. Worst the scale is so large an entire raft could break away and float along with running water or just out into a lake center.
Giant Lotus as Combat Terrain - You totally want there to be GIANT MAGIC LOTUS with like house sized leaves. That way you get complex springy multi layered platforms that may or may not be full of temporary rain water pools on the verge of spilling over into a water fall and cascading down lower teirs of leaves to the water. The general terrain fun of GIANT LOTUS is HUGE. You could have cities under or AMONG those leaves, it would be great.
Mythical Lotuses - There are TWO species of lotus in the real world.
Nelumbo nucifera and
Nelumbo lutea. There are... a lot of pretty hybrids, hundreds maybe more. But there USED to be eight SPECIES of lotus. They are all extinct now. The closest genetic relative remaining in the mean time is the
London Plane Tree. However EVEN IN THE REAL WORLD there are NUMEROUS completely fictional sorts of lotus. From the poorly defined stuff of the "lotus eaters" to well known things like the "Black" and "Purple" lotuses of Asian mythology (which are almost always poisonous, medicinal or outright magic) or the most famous, the Blue Lotus of the Nile (almost certainly actually a mistranslated
Nymphaea caerulea that possibly really DOES have some mild narcotic properties and an indescribable subtle but very very sweet, sort of addictive, scent). So OF COURSE you have the cool mythical lotus plants and such in any fictional world. Hell, I'm pretty sure there are "lotus powder" poisons in core 3E.
Magical Lotuses - GIANT MAGIC LOTUS, also, super addictive drug lotus, also, it needs to grow in the rain, also, golden lotus, purple, blue, black, and hell, a TRUE red just so the Chinese breeders of the last three millennia will FINALLY be happy.
Papyrus
It's not a food crop, you grow it for paper, fibers, and flotation devices, ask the Pharaohs, they'll give you the details. But for most purposes treat it like lotus that looks different, and which has it's root raft much higher in the water and is MUCH more likely to raft out over deeper water or form raft islands. There are lakes in Africa full of floating islands formed of lesser water weeds stuck to Papyrus super structures. These islands constantly shift and grow and break and move into an endless shifting labyrinth. Western explorers got seriously lost in this stuff, taking months to travel almost nowhere due to the confusion of the stuff. It's lots of fun.
Other Things There are countless much more boring food plants grown in water. You don't care about them, commonly they are just some funny leafy vine boggy thing that grows in the irrigation ditch that farmers just happen to also harvest to throw in their salads. Other times they are something fun like water chestnuts (both sorts) or that plant in India which you can carve almost anything out of and which gave the Pith Helmet it's name. But generally they are just an "extra" plant or somehow less interesting for role play purposes. I mean, unless you are carving elaborate Pith Armour or something...[/b]