Great little game. Just got my playdate last weekend and this has to be one of my favorites so far. Am I right in thinking that it takes inspiration from Link's Awakening?
Only suggestion I would offer is a nice QoL addition would to add a "are you sure you want to leave the island" prompt when interacting with the boat. I had done basically everything on the island (I assume), but hadn't interacted with the boat yet. So when I did so to see what it did, I accidentally ended the game with only 2 seashells left to find to gain access to the pirate treasure. As a result I have to start all over to actually complete the game.
Other than that, 10/10 I won't really mind replaying it to see what's behind that door.