If the water stays hot like you say, it's likely you're seeing a back/cross feed. Expansion issues typically will feed hot water for a minute or so, then it'll cool down. Most marine water heaters are installed with a check on the cold supply line.
Fortunately, because of the temperature not being where it's supposed to, it should help in troubleshooting. You'll have to be methodical, and step through every line. Start by duplicating the condition, then work through the plumbing and note each place the cold water isn't cold. I'd start at the water heater inlet. If there's a check valve, there's little potential for back feeding through the check valve. If it's hot on the side of the check furthest from the water heater, the check is probably blocked open. If so, remove it and free it up. If it's calcium buildup, just dunk it in some muriatic acid for 15 minutes until the calcium's gone. Even if there's backfeeding, unless there's a source for the cold supply to jump to the hot side somewhere in the system, it's not likely to stay hot very long.
If the check isn't hot, then follow the cold line and see where there's a hot spot. If any fixture has a hot hot line, and a hot cold line, suspect a failed mixing valve that's cross feeding hot into the cold. It's unusual, but possible. If there's a deck shower with a mixing valve, check to see if the hot/cold valves have been left open and the shower head valve is keeping it from flowing, that's a good spot to check.
Be methodical, be thorough. Check every fixture. The one you overlook will be the one that's the bogeyman!
