Thank you for the fast response!
I don't think it's the water textures - it seems to be the pathfinding for the fish, because if I reduce the regional tiles that they can swim around in, the FPS improves.
I also have one more question: Is there a way to increase the max casting distance above 10? Right now, if I set it to 20 and the rod's casting distance to 20, it still snaps back to the 10 distance after casting.