I would think that a 2" line would suffice, but I'm just guessing. Each boat = 2 bedroom 1 bath house (sorta), depending on the number of slips on a dock....
It also depends on the pressure and main size. Probably need an engineer to do the math.
Remember that generally they drop the pressure at the hose bib, so the line to the boat isn't going to bust a gasket somewhere aboard the boat.