Flooded vehicle

I have a 2004 GMC Envoy and I attempted to cross a creek in it and the
water was deeper than I realized and as I pulled into the creek the engine
stalled and I immediately tried to restart it but it would not even turn over. My
first assumption was that maybe the water splashed up and got the
alternator wet or something like that so I called for a rollback to come and
pull me out. It took about an hour for the rollback to arrive and by the time it
got there water had seeped into the floorboards approximately a foot deep.
Parked it at my parents house and I let it set for about a week and had my
parents neighbor who is a mechanic look at it. He said that there was no
water in the oil or transmission so he tried to start it and told me that it acted
like the battery was dead. Waited about 2 months and he went back over
there with a battery booster and tried again and he said that all the lights
come on like they are supposed to but still would not turn over. What do I
need to do to get this vehicle running again?