Boats don't Float to the surface if spawned 2 or more blocks under water. Such a boat sits there or sinks to the bottom. The player can't ride these underwater boats. The player can get in the boat but is immediately removed from it. The message "Slow down to exit the boat" appears and fades away slowly, but the paddling controls and the "Leave the Boat" button transition out so quickly it's nearly impossible to notice them.