If you are hit by a slow moving boat, your character becomes unconscious and ragdolls all the way to the sea floor ... and regains consciousness after hitting the sea floor.

The expectation is that the player would drown quickly, due to the fact that he's not holding his breath while unconscious.

If the player has a rebreather, it's likely that the mouth-piece would slip out after being hit, and becoming unconscious ... thus drown quickly as well.