People like to do this alot. I see now need in it.
How many hours a day while motoring do you look at the gauge? why arn't you looking where you are goiing?
An over heating
is a minor incident. yes with a gauge and constant monitoring you can catch things ahead of time. but I'd rather sail and fix it when it breaks, not wory about when it's going to break.
Just my $.02
To answer your question. No I don't know of any. Just get a gauge with a 3/8 MIP thread sender and use one of the ports or make a T to go in line to the water heater
for the sender.