Not much. Cruise line is planning to run a number of ships on liquefied biogas.
IMHO the pollution at point of use is going to be pretty low if everything is working properly, but I wouldn't want to live downwind of the bioreactor that's basically rotting fish, or anywhere near where they dispose of the spent sludge or even be on-board if there's even a small gas leak from the fuel system, as the stench is likely to make you reacquainted with everything you've eaten in the last week.