By expanding in from another board, it's thus possible to make an Ocean that's split into two parts: the printed Ocean and the bay formed from the Ocean on another board. Since the two parts make up a single Ocean, they're one land any time the Ocean is treated as a land, all pieces in one are also in the other, etc. While it may be more thematic for each land to become part of the Ocean it was sunk from, playing that way ends up raising a bunch of extra rules questions and requires extra tracking.