Nice game, really liked the swarm mechanic, and how you need to circle around if you want to keep your fish grouped.
As others have pointed out, some tutorial or hint on how to destroy boats, and some more feedback (e.g. boat hp) would definitely help to understand the game faster.
Also, the background sometimes makes it hard to know how fast and in which direction your moving (i know the fish follow the mouse, but sometimes it doesn't 'feel' that way), and it's really hard to get to the last boat (lost a lot of good fish on the way).
But overall, really cool!