I can tell you my 27' jumbo is 5500 lbs on the trailer axles, probably 300lb tongue weight, this is fully loaded for a week long camping trip. Trailer weight is 1,300 lbs on the axles, 50 lbs tongue weight. 4,500 div. by 27 this should give you a good per foot weight on a jumbo.
Salt water use I highly recommend galv. or alum. trailer.