>>1492279If you are moored in a marina you just plug yourself into the mains and connect your water tank to a hose. You have to pay for this though.
If you are on the river/canal network you get electricity from running your engine, this charges your batteries and you plug in your appliances as normal (most people have solar for a bit of extra power). The British Canal & River Trust have water points all along the navigable river and canal network, these allow you to fill your water tanks and empty your toilets. Most boats have a shower, bath and toilet on them.
Some guy doing a tour of his boat on youtube:
https://www.youtube.com/watch?v=hPiofQl4PAIhttps://www.boatsandoutboards.co.uk/Motorboats/Canal-Narrow-Boats-for-sale