While relocating the fresh water manifold I saw that the water heater was showing lots of rust on the bottom. Turns out the unit was an original Atwood EHM6-SM so way past replacement time. After doing some research I wasn't too excited about paying $400+ for the drop-in Seaward which uses the same obsolete design as the Atwood. Some more research pointed me in the direction of Isotemp water heaters which have a stainless steel tank, are better insulated (750W heating element vs 1500W) and don't cost that much more. They also come with a mixing valve to set the temperature of the water coming out of the heater which has the added advantage of making the hot water last longer.

The only question was whether I could fit one in the space available. The SPA 20 (5-gallon) fits pretty well and leaves enough room to route the coolant hoses to the front of the unit. Looking forward to reliable hot water on the boat!