We need some non boss level enemies on the ocean. Why not shoals of mermaid skeletons that jump on your ship and attack the crew?
You can try and steer enemy ships into them, there could be a mermaid leader that would drop a skull as a reward, and I'd say they despawn after a certain point if you die to them a few times so newbies aren't ruined by them. Plus it would make the longer voyages more interesting if you could do something without stopping your journey.
Main thing would be they wont sink your ship, just attack the crew.
Feel free to discuss ideas and upvote this if you like the idea!
