The biggest difference between a marine engine and an automotive engine, from a block/structural point of view? The marine engine has brass core plugs. Lakewater doesn't have corrosion preventative chemicals in it, and saltwater is hatefully corrosive. Brass core plugs last longer than the more common steel plugs.
Well, maybe it also has a little more piston to cylinder clearance, too. At least, that's how I would build a marine engine--GENEROUS clearance, because the cylinder wall stays pretty cool in a boat that uses lakewater for cooling. Since the cylinders are cool, they don't expand much. Pistons run about the same temperature, though, so they expand just like in a car engine. Result--you need more piston to wall clearance to prevent scuffing.
As has been alluded to, the entire GM performance crate engine program began as a result of creative marketing of the marine engine product line.
There are other differences between a marine engine and an automotive engine, but it has more to do with accessories than the engine proper.
Cylinder heads will often--but not always--have extra-hard valve seats and extreme temperature (Inconel) valves. Reason: Performance car engines may see 6000--7000+ RPM bursts, but boats will run for hours at 5000 rpm/full load. The heat load in a boat is incredible, but you wouldn't know it to look at the coolant temperature gauge. Oil temperature gets insane in a hurry if you don't have an oil cooler.
Marine intake manifolds often have a brass insert for the water passage/thermostat opening.
A boat has a solid hull under the engine, a car is open to the atmosphere. Since fuel vapors collect in the boat hull, the Coast Guard requires carbs that are sealed to prevent fuel from leaking out the throttle shafts, and the bowl vents positively vent into the venturis. No paper gasket between the carb and spark arrestor to absorb fuel vapors and catch fire. The distributor and starter are special to prevent sparks from contacting fuel vapors, and the fuel pump will have a clear tube leading from the fuel pump vent into the spark arrestor and directed down the carb venturis. All these modifications either reduce fuel vapors, or the sparks that could ignite the vapors. The exhaust must be water cooled. On "factory installed" engines, this usually means water-jacketed exhaust manifolds, but "hot rod" boats just spray the water into the exhaust stream via small nozzles in the header primary tubes. (Wet Headers)
The wet headers work very poorly at idle--you may get enough exhaust reversion to bring liquid water in contact with the red-hot exhaust valves. You can guess the result. Therefore, some folks install wet headers with a throttle activated shutoff--the headers go dry at idle. I don't know if that's legal by Coast Guard standards.
