This was pretty fun! And as someone who also programmed 2d waves, I appreciate the effort involved.
Three things to consider improving: (1) Jumping the boats is a lot of fun; probably the best part of the game. You should get bonus points or a speed-boost for doing it. (2) If the AI dies from a shark, they should respawn. Having the game end because the AI is dumb doesn't feel great. (3) It's possible to get stuck on the boats. Maybe the dolphin needs a way to flop/jump off.