Odd, as the only way the pump would cycle is if it loses pressure. If it's not going anywhere into the boat in the form of a leak, the only place it could be going without leaving a trace is back into the tank, via the pressure relief valve on the hot water tank. Have a look at that. We did have a failed one that was leaking, it was under a tenner to replace from a plumbing supplier. It's the one with a red plastic cap and the pressure where it opens is written on it somewhere. You can take the return pipe off and see if this is where the water goes.
We fitted a similar thermostatic mixing valve to the hot water tank and no problems.
