Great Game! It was fun to play, but i have few suggestions/problems with it.
First is the UI/GUI. You can get lost there easily and i suggest to simplify it. Main menu is mess and seems un-responsible (my suggestion is to: simplify and add animation to buttons).
Second thing is boat physics. You can too easily change direction in which boat goes and it has weird feeling that it accelerates if you press two buttons at once like W and A or S and D.
Third thing is waves. I suggest that you spawn them not every enemy at once, but over time as it can get laggy (experienced it).
Fourth thing is leveling. You can easily miss upgrade you want when you're fighting enemies.
Fifth and last thing is music. Game could use pirate-themed music in background.
Overall game is great, but needs improvement
(sorry if something isn't understandable, it's because english isn't my main language)