As with almost all of my jet boat knowledge, what I read was...
The cylinders on the marine engine need to be looser than a car engine, the cylinders get hotter in the marine ap. therefore expanding more requiring looser cylinder wall to piston clearences. So if that 350 is very well broken in you should get at least one full season out of it. If it's fresh, expect it to expire quickly. Hull design, hull weight, and driving practices will play a huge part.
I've got a friend that had the most reliable boat of all of my friends, a completely stock, broke in is a huge understatement, probably 100,000 mile 454 that lasted 3 seasons until he tried to show off on a sand bar pass throwing a engine rod.
I'm going with a car 454 I bought off of craigslist, easily a 60,000 mile engine, I just won't be showing off because my pocketbook won't allow it. I wish you luck, I know that's what I'll be counting on.
