Awesome concept! I love the idea of having to navigate using only landmarks. The map feels the perfect size for the length of the game, as I was finally mastering where each port is at the end. The shark additionally forcing you to change direction and potentially lose your place in the dark is really good. I give 5 stars for innovation. The shark model was really cute, (and really scary at night), but I would love to see some better detail for the islands and the rest of the game. Also the shark fight wasn't as engaging as it could have been, but it was a perfect ending to the navigation part of the game.
If you wanted to expand the game, maybe add map fragments and new ports to be able to go further out to sea.