The Wolters on-demand propane water heater was specifically designed to provide hot water on small sailboats. Gerry Wolters, the inventor, used it first on his own Catalina 22. After it became popular, he sold the company, but I believe the new owners didn't make much of a go of the business. A few years ago, Gerry formed a new company, called Precision Temp Inc., which started making them again, and also started marketing them to RV builders, which significantly enlarged the market for the device. I believe Gerry's water heater is the only device of its type on the market in this country that works well.
I used to sail with Gerry and his wife, Elaine, in those early days, and they were nice folks. You can log on to the Precision Temp website at the following hyperlink: