Campers seem very happy with them. As TrueBlue said, you've got to be careful about propane and exhaust on the boat. Propane bottles are basically sealed with a rubber ball on the valve, and sometimes they simply LEAK. Assume that yours WILL LEAK and use and store it where the leak can't do you any harm. Similarly, flames produce carbon monoxide, so I'd restrict the use to a reasonably well-ventilated place (like your cockpit).
It is just a small electric water pump, and a tiny hot water heater. A very nifty combination if you just want hot water for a short shower once in a while! There are a number of companies making and selling them.