Our dream is also to retire "on the water". We were just in Luperon D.R. There is a protected bay where many sailboats and the other kind are mooring. The cost for the month is $15 USD. The cost of living is extremely low there too. There are yachts for sale there also. See Luperon Boat Yacht and Property Sales - Caribbean Yachts - Dominican Republic
Check out my pictures at Picasa Web Albums - Jamie Miller - :Luperon 2007
I am not saying this is the best place but it is one of many.