The sea could have interaction with the seasons, or winter, it could freeze, or create some frozen paths, and some islands would be impassable because of walls of ice, for example the lunar island would not be accessible in winter, that would also provide an explanation of how Deerclops and other bosses appear, they come from a place beyond the sea, traveling long distances.
Maybe deerclops only appear in winter due to the fact that winter provides paths in the ocean, or maybe it will come out of its hole in winter and go swimming there. The world beyond the sea
Just a story