I took a 24V positive feed from the 24V supply that's on the forward wall of the engine room to a circuit breaker mounted in the same box as the watermaker cct breaker. From the cct breaker to a relay which is controlled by the timer. From the other side of the relay to the positive feed to the FW pump in parallele with the positive feed from the galley cct breaker for the FW pump.
I switch off the galley cct breaker when leaving the boat and set the timer to Auto.
No, there is no interference with the normal operation of the FW pump.