In my extended cruise from NW Florida to Freeport GBI, something I
have seen repeatedly is that marine heads are a major PITA for people
(thank God all I have is a porta-potti on my 28' boat). Even the huge
expensive boats seem to have problems getting pumped out.
So, if you do not mind getting into ****, you could probably finance a
cruising life by knowing how to deal with such things. Just getting
marine pump-out facilities to connect to yachts could pay for itself,
carry some adapters for the various systems and you'd do well. From
what I can tell, a guy with a million dollar yacht and his wife (who
really does not care for boats) bitching cuz the head is backing up
cuz the holding tank is full will pay just about anything to get her
off his back.
So, hang out in expensive marinas where there is ussually good eye
candy while you are connecting the stuff nobody else wants to deal
with to pay your bills.