When you have at least one man on the assault boat lying at the front, allow to deactivate the engine and let the guys paddle to approach the beach silently, like Special Force guys do.
Mechanics would be like this: When boat isn't moving (engine on or off), the player lying on the side of the boat can just hold down the forward moving key and the character will do the paddle animation. The more players do this, the faster you go. Driver should be able to lay on boat (if there's place) and help.
If all players release the forward key the boat stops and floats.
Also, please allow to aim and fire when lying on boat, also the driver should be able to just select his gun and fire, but can't steer when he has his gun at the ready (I honestly didn't try that, sop it's the case already in the game, than please accept my apology).
Hmm, maybe allow the driver to use the crouch key to go lower (lower profile, duck from bullets).