Our engine-heated hot water gets too hot to put your hand in it, so your problem likely stems from a fouled exchanger within the 20 yr old tank. (ours is only 4-5 yrs old)
We occasionally use the shore power for this purpose, but motoring for 1/2 hour or so each day provides us with a heated tank that lasts the day unless showers are required (we rarely use the shower - preferring solar heated bags used on deck)
After sailing for a number of years heating water on the stove, it's sure nice to have a ready source at hand. If its not leaking I wouldn't be in a rush to remove it.