not a whole lot to do (like you said) but i like this take on the theme! that wave is quite scary, indeed D:
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one other idea for the sake of progression I came up with a while ago was the wave washing up a bottle of some kind of potion that allowed you to 'level up'. It made the game harder and easier at the same time. Though critiquing my idea I could have made the good side of leveling up through washed up items that could help with progression. Dang maybe I should finish this project 😅