IMO Raritan water heaters are best quality
They have a polymer shell that can't corrode...are exceptionally well insulated, so they hold water hot for a day or more...and they have a glass inner tank that can't corrode either. They cost a bit more than some others, but are the only ones that typically last upwards of 25 years. I've owned two boats with water heaters--both Raritan...one was 11 years old and still going strong when I sold the boat, the other was 21 years old and still in excellent shape when I sold the boat...and that's typical. I've never seen a household water heater last more than 12-15 years, much less any other marine water heater. I wouldn't have anything else. You can read all about 'em here: http://www.raritaneng.com/products/water_heaters/index.htmlThe only kicker MIGHT be whether one will fit your space...so check the dimensions for the various sizes, with and without heat exchanger option, very carefully.