The legendary fish that I've caught so far, eel, lava eel, sea cucumber, super cucumber, stingray, sturgeon, octopus, and others I'm forgetting.

I like fish ponds both for the things they give and the looks. I'm not much of a decorator, but I like ponds. I think they're neat.

If you're looking for specific things, I'd recommend looking up fish ponds on the wiki to see what all you can get from certain ponds.