Seems odd to me. I've launched a bunch of boats and driven lots of manual tranny vehicles. I've never used reverse to back down a boat ramp. Did his foot slip off the clutch at the same time his other foot slipped off the accelerator while in reverse for a procedure that most folks use neutral to let the weight of the boat and truck ease back into the water while holding the brake? When I owned my '03 with the driveline vibration that DC couldn't and wouldn't fix for two years and 44k miles, I thought about drowning it many times as I drove past the boat ramps. I'm not making accusations, I've just been reminded of what I wanted to do with my problem truck.
I think as it was the end of the day he was positioning the trailer and then go of and get the boat or someone else was in the boat waiting and once the submarine commander finished his dive, moored the boat up out of view and called for help.
Just a quick correction, should that be a U-boat not a submarine as it's a Dodge?