Congrats, you're the first person I game 3x5 stars to! If I had any suggestions, they would be these:
- Making an in-game tutorial;
- Making "2" go down and "5" confirm in the harbor menu (I mean, on my keyboard numpad "2" literally has a down arrow on it);
- Improving the UI in the harbor somewhat (making the best use of space, making sure text doesn't clip into other UI elements - it does in one of the upgrade options, I forgor which)
- Making unique sprites for the different crates and fish in-game - I know they have different appearances in the shop, but it would be nice if there was some more variation in the game view as well;
- Ship sprites in increments of 22.5° (π/8 if you're a nerd like me) so it looks more smooth;
- More distinguishing features around the coast, like houses etc.;
- Enemies with different attacks or attack patterns;
- Some actual original music using the soundfont in the resources - even just a uniqur jingle in the splash screen would be nice.
Keep in mind that although this list is long, these are all just nitpicks, things to think about for a post-jam version. You really impressed me!