Talk to the guest of honor. They can narrow down which ideas resonate with them. If it is a young kid, the mythical sea cakes (mermaids, pirates) might be a good bet. Or if it is for a baby shower, any of the sea animals can be made to be cutesy or gendered. Alternatively, an adult birthday celebration or retirement celebration might call for a sand & surf-themed cake. My particular favorite is the minimalist sea shell cake. However, I recommend talking to the guest of honor before deciding.