Since the first game's version, when your boat (assault boat) simply respawns (with the vehicle respawn script v1.7 or v1.81) in a edited mission, you are not able to drive it, or get in, because the boat commands missing!!
Then, it's like a static object, that you can't touch or drive!
Sometimes, we can resolve this if whe have a engineer with us that can repair the "new boat"(<=> 0% damaged!!)...
Only with engineer but not always, we can take the boat another time, if he repaired it before...
... and we have to do this each time the boat respawn!