First of all- well done on your first game project!!! **applause**
This is a really good two-day game, even for a seasoned indie, so very well done indeed! The artwork is really cute! I really like how you constructed the environment to have water and islands, with different benefits to each one. And it's cool how you drop down into the water too~! You even found time to add different enemy difficulties! :3
The only constructive criticism I'd give is HP regeneration was a little confusing/difficult. The way I understand it.. you gain HP for destroying enemies, but only if you're in the water at the time, right? Whereas on land you run faster but can't regain HP, something like that? And I think 1 enemy that takes 2 shots to kill gives you your 2 shots back, right?
Assuming I've got that right, the advice I'd give would probably be.. just be a bit more generous with HP regain, since one missed shot, or a shot on dry land, will be lost forever. I would either make enemies give back a little more than you lose killing them, so you have a chance to get your lost shots back (while keeping the 'you only gain when in water' mechanic, which was a nice touch!). Or make the water regenerate your health slowly over time, regardless of if you've shot enemies or not, that way you're essentially risking slow movement for HP regeneration. OR dot some HP around the level for players to gather - so they have to explore the map while shooting enemies to find them. :D
That's just my advice / an idea that I thought I'd give. Feel free to use or ignore it ^_^
Again, very well done! <3