1st step is to charge the battery. While the battery is on the charger, using a hose, flush your alternator, starter, fuse box, and all electrical components, wiring, etc with fresh water. Same with everything on and under the dash. Let it all dry for a week to 10 days. Using a fan will help to dry things out a little sooner. Install 1 battery. Turn the key, see what happens. You might be lucky, but maybe not. It's going to be an uphill battle for sure. I'm guessing that the ECM, TCM, instrument panel, will likely have to be replaced. You'll find out soon enough.
I used to do some saltwater fishing with a friend in his boat and there was always salt water in the bilge, in the starter, and sometimes in the alternator. Kinda went with the territory. But as soon as we got home, we flushed those components with fresh water. Never had any problems with the electrical components because of the saltwater. It was fairly common for people to launch their trucks, vans, etc while launching their boats. More times than not, they would start and run while still on the launch ramp.
2008 Silverado LT, CCLB, Edge Insight CTS
2009 Arctic Fox 29-5T, Silver Fox Edition, Onan 3600 LPG generator