Has potential =)
It's good as it is but it has the potential to be expanded :)
From a brief overview I thought of a way which you could do it.
- Create different level fish and give them their own health bars.
- Instead of rewarding with points, reward money and design a shop.
- In the shop give them different boats, guns and power ups.
- As the level of the boat increases you can provide different rates of speed and defense
- As the levels of fish increase, increase their damage and reduce their encounter rate.
- As the levels of the weapons increase, increase the damage they do and possibly implement special attacks.
It's pretty good for a basic game and these are just a few pointers if you ever wanted to expand / redo it =P