It’s behind the panel in the aft bunk on the port side. You should find not only the water heater but the fresh water pump also. Not a big deal to remove. MIne has 8 or 10 screws with finish washers on them and an AC duct which pops right off.
Fyi, on my Marlin there is a strainer attached to the fresh water pump inlet. Doesn’t hurt to pop that off once in a while and clean it. I’m pretty meticulous about keeping my water system clean but you’d be surprised what still accumulates in there.
Tom