Right now, the only dangers in the water are leeches and drowning. If you have a pair of water breathing characters, you’re set. Now, leeches can get annoying but they’re still not very dangerous. I suggest something like a shark, that will eat from the giant fish swarms and will leave the bigger fish right there, ready to collect. Basically the bearenya of the water, but you wouldn’t be able to sense it other than seeing.