Ok - you got me interested enough to Google it...
there are a lot of interesting facts and figures on Diesel use in the US HERE: L2_1_1_rf
Here is the pertinent section;
"FUELS FOR MARINE DIESEL ENGINES
A variety of fuels are available for marine diesel engines. There is a set of four marine distillate fuels, some of which contain small amounts of resid,6 and a set of 15 marine residual fuels in which resid is the majority constituent. Since resid increases viscosity, these fuels range in viscosity from less than one centistoke (cSt) to about 700 cSt at 122°F (50°C). The higher viscosity grades are preheated during use to bring their viscosity into the range suitable for marine diesel engine injection (8-27 cSt). Marine fuels also contain more sulfur than on-road diesel fuel. The maximum sulfur content specification varies from 1% to 5% mass for different grades.
Several organizations issue marine fuel specifications. Standard 8217 of the International Standards Organization (ISO) is the primary standard. ASTM maintains a parallel specification, D 2069. "