Hot Water

Gas

 

Back Boiler on your wood burner

This hot water tank (called a colorifier by boaty types) is filled with hot water that is heated via the back boiler on the wood and coal burner.

Engine and Calorifier

 

Solar water heating

solar hoit water system

Has any one fabricated,or thought of heating water via solar panel(s) and a tubular tank on the roof of a narrow boat?

I've seen a coiled black

I've seen a coiled black rubber pipe used as a solar heat collector before now. It's basically a black hosepipe, preferably angled towards the sun, that's filled with water. When the water is warm enough, open one end and use it. Using PV panels would introduce lower energy collection and 12v heating isn't much use anyway.

https://www.youtube.com/watch?v=1tTtPLGjOJo

There's a trade-off between the amount of water you want heated, and how hot you can get it to. For example, if you have a small amount of water that absorbs all of the heat, it will get hotter than a larger amount of water. To get more, hotter water, you'd have to increase the surface area of your collector. It's also totally reliant on the sun and how much energy from it is able to reach the collector. In northern latitudes, you'll get less energy from the sun anyway, and so need a larger collector.

It's definitely a thing that can be done, it just depends on how much room you have, how much water you want to heat, how hot you want it (don't forget legionella!) and where you are in the world.