I believe the ship moves in the same way as a lift. the passing water is a texture that gives the impression of movement. When you move from one stage of the event to another all the dead bodies “jolt” as the lift/ship stops ( think after chaos ships before skaven ships) , I imagine it’s a way of stopping the barrels getting lost overboard and not respawning as they aren’t destroyed.
On the subject of barrels, maybe there’s too many items in the hold and makes it harder to pick up the barrels? Keep swapping bombs from fire/blast/fire/blast instead of pick up the barrel is a bit maddening.