Dude, knowing just a smidge about boat recovery, I really wish ya luck in pulling it out of 15-20 ft of water w/ just a 8k winch.
Ideally, you'd wanna get some air bladders on front and rear, and the low side to flip it wheels down when all bladders inflate. Then drag it near the edge, get it resting on it's wheels, then pull VERY slowly up on terra-firma. A boat would need a few high output bilges running as it reached surface, but the truck should self-bilge w/ just opening doors if ya pull it out slow enough. Pulling out too fast will quickly overtax even a 15k winch. Have two divers in the water, and a 3rd ready to rescue one if the rig rolls wrong and heads bottomwards.
I'm making this a big production, but I don't wanna see your buddy post up that you joined your rig in it's watery tomb. There's a real finesse involved in righting the rig while keeping it outta the rocky bottom, while fighting a current. Water is stoopid heavy, so be careful.
Good luck.