Yep, it's AC/reverse cycle heat. A circulation pump draws raw water
in via a (usually dedicated) thru-hull, and that's also disharged at the rate described. Depending on thermostat settings and outside temps, the system could run either continuously or periodically. Usually said to become very ineffecient for the heat side once surround water temps get below about 46° but I don't know that for sure (would have assumed at least some heat would stil be available if raw water
temps are lower than ambient air temps).
Some boats take on a lot of bilge
water, but usually a bilge pump
output -- which could look ike a similar flow volume -- will only run for a short time, i.e, until the float switch returns to rest position. Usually wouldn't be genset running in a marina. AC condensate, dehumidier, and even shower sump
output is puny, compared to AC/heat raw water flow.