What is the opposite of raft?

The opposite of the noun raft could be "submersible" which would keep you under the water rather than above. Or possibly "ballast" (weight to keep you underwater).

More arcane would be "pool" which is a water above a dry surface, while a raft is a dry place above a water surface.

*The less frequent use of raft to mean a large unspecified number (e.g. a raft of problems) could have the antonyms few or countable or numerable.