If you have insurance
Step 1 - file a claim
Step 2 - let them haul it away
If you don't
Get the engine/trans drained out asap, pull the plugs. Put fluids back in, crank it over to get any water out of cylinders. Then put plugs back in and start it up. Then you need to keep draining and refilling the oil/trans fluid until it looks decent. Don't stop until you have gotten it all in a good state and can run the engine normally. Whatever you do, don't drain everything out and let it sit or the engine will be junk.
You are going to need a new starter, new alternator and you will be chasing electrical gremlins in the wiring and dash for the rest of the time you own the boat. - So you may just want to set it on fire now and save yourself the trouble.