Car motors have different piston/wall and valvestem/guide clearances. The cams aren't the same,valve materials(especially the exhaust) aren't as good,the oiling pressure side and drainback sides are not usually modified as a boat should have. And most of all,read the fine print,it usually states"not for boat use".This is because they know that boats require the abovementioned differences.Boats run at cooler tempratures,higher power levels and rpm for longer periods of time, I have just touched on the basics,consult a REPUTABLE BOAT ENGINE builder for more details.