Marine Diesel for recreational boats is off highway #2 diesel oil. Pretty much the same as highway use except it has a higher sulfur content (except for northern California which uses ultra low sulfur highway fuel
edit: I forgot...as of June 2012 marine diesel is ULSD as well.....I blame the Sailor Jerry. I add 2 stroke oil to lubricate the HP fuel pump
and injectors in my old M-18.