A while ago I fitted two tank gauges on Chestnut
They are brilliant. One of the most useful things I have fitted.
They replaced two wooded sticks. Each stick was dropped through the filler hole to the bottom of the tank and then removed. One then had to guess how much of the stick was wet. Like the dipstick on a car. This is fine for the fuel tank but was a bit dicey for the water tank.
They are made in the UK by MCS boat products:
Here is a photo of the Fuel Gauge Sender unit attached to the bottom of the fuel tank.
Originally this was the drain cock for the fuel tank, but the addition of a service valve, all done with the tank empty, allows the sender to be removed or serviced without draining the tank.
Another photo showing the connection wires to the sender unit. There are three connections, so an ordinary mains flex is ideal and cheap.