Very interesting brainteaser, thank you!

http://regard-house.ru/3dplan/test/boat-no-water.html



1. Draw room with hole around your boat, make it water texture. Level thickness should be large, as the depth of your sea.
2. Make walls half transparent for a while (Modify 3D view/Walls transparency), and your sea becomes also transparent
3. "Copy as a furniture" (this is plugin) the sea and any smallest furniture together (you cannot copy this way only room without furniture) and paste it. Export to OBJ and import furniture may also work, I did't check.
4. Make walls nontransparent back.

Yours, Daniel