While you are playing with the water heater plumbing
, check that an isolation valve is fitted.
Our L400 had no way of turning off (isolating) the fresh water feed to the water heater. If the water heater decides to leak, the only way to stop the flow is to turn off the boat's whole fresh water system. A cheap
valve fitted to the fresh water intake of the water heater is an easy fix. Just a thought.