We have a Junkers propane hw heater onboard, as originally installed by Nauticat Yachts. It's a direct vented, bulkhead mounted unit in our galley and has been upgraded with all the ABYC safety devices. I have posted here in the past, about the benefits of this system while away from shore power and cannot say enough about how my wife and I love having unlimited, VERY hot water on demand. Our 140 gal hw tank adds to this luxury. I have no issues what-so-ever with this system.
Junkers was a German manufacturer of precision water heaters for decades, until acquired by Bosch a few years ago. Bosch used the Junkers name for a while, but I believe discontinued the name while utilizing the design. The units are very popular in Europe and are used on some high-end boats in the US. A quick Google search led me to this site, but other distributors have them as well: