It is 5.8 liter (that attitional cubic inch puts it over the edge) actually it was probably a marketing thing. Like why would you buy a Chevy with a "wimpy" 5.7 liter when you can get a "mucho macho" 5.8 liter in a Ford? When did the big three go to the metric system? Anyways, the MPI EFI off of an old truck or van would work fine on a boat with the only issue being the intake manifold height, you know would it fit under the motor housing. A 351w in a boat has the same bolt pattern for heads, intake, and exhaust as 351w in a car truck or van. The heads from a 302w also fit a 351w and there is only one head casting, you just flip it around for the other side of the motor. The lower (in an EFI situation) intake manifold is different between the 351w and the 302w as the 351w is actually wider where the intake meets the heads as a function of the taller deck height. The upper intake manifold is the same and used with a specific lower like a stock upper and a stock lower or a gt40 upper and a gt40 lower. There are plenty of aftermarket EFI intakes available for windsor motors both the 351 and the 302. The MPI EFI system itself exists in a couple of different forms from the factory. The older system is called speed density and uses air temp and throttle position to determine its fuel tables. This is a very reliable system, just not performance oriented. The other factory system is mass air which uses the amount of air coming into the system as well as temp and throttle position to determine fuel tables. The mass air system is deffinatly the way I would go with a boat. Here is a short list of what you will need.
EFI manifold upper and lower
Mass air meter (has to match the injectors #/hour)
Injectors (probably 24#'s for a 351)
Fuel system which includes lines, rails and a pump
The brains for the mass air (ECU from car or truck)
Engine wiring harness (for injectors and mass air meter)
Misc. wiring and gaskets
From a junk yard I would say you could pull this all off an F-150 for $500. IF you go the buy it new route, aftermarket manifold, Ford EFI conversion kit, new fuel system... you are looking at an easy $2500 and that can go up from there depending what you want. FYI the fuel system in my mustang was $2500 (tank, pump, lines, rails). The moral of the story is sky is the limit, how fast do you want to go?