Could a regular auto mechanic work on a marine engine?

I have a 1989 Thundercraft Bowrider. The engine is either a 3.4 or 4.3 L... It is basically a Chevrolet auto engine, according to any mechanic that has ever looked at it. It has 188hp, if that matters. I have a very good friend who is an excellent auto mechanic that works on my Jeep and my truck. He can fix anything auto... fully certified, etc. etc.. I would love if he could work on my boat, but if it is a bad idea, I won't ask him. The reason I am asking this is that my experience with marine mechanics has been very bad. Also, I spent last winter and part of this winter overseas, so it was not winterized. My brother claims he drained all of the water out of the hull and The climate here dips below freezing, in the low thirties, about 50% of the time at night during the winter (northern South Carolina). HELP! I want my boat working!