Guess I would want to look at the specs for marine motor oil (assume 4 stroke engine here). I suspect marine motor oil is going to have more antiwear additive, that is, it's not subject to EPA catalytic converter warranty requirements, nor CAFE, so should be more robust like high mileage oil. Actually, a HDEO like Valvoline Premium Blue or Shell Rotella might be better, since not all high mileage oils have higher levels of antiwear additive.
You maybe should post the question under the "Boat/PWC Lubrication" forum at this site. Those folks will be much more familiar with marine engines. In fact the answer may be very apparent there, just by browsing.
