The hot tub you are looking for is called a "Tublicious" Made in Vancouver BC.
I have one on my 4788 fwd on the boatdeck. It worksvery well on the boat with shore power hooked up. I normally empty it for a trip BUT I have run with it full and really did not notice much difference in the roll rate. I would never go on a long trip with it full but just hanging around the Gulf Islands is no problem.