Car engine compartments are open and vent out fuel vapors. Cars with fuel leaks will start on fire without the explosion. Boat engine compartments a mostly sealed and vapors from a very small leak can have tragic results. If he wants to use car parts on his boat, then feel free to ride with him. Just don't expect any sympathy from me when tragedy strikes.
Marine alternators, distributors, and starters are designed with vent screens to reduce the risk of explosion. If fuel vapors accumulate in an engine compartment and there is a spark, backup and watch fireworks.
Why do you think they have bilge blowers on boats?????
Also, your insurance will deny any claim.