Water pump causes led lights to flicker when on Battery

The battery monitor isn't going to react fast enough to show you what's going on. Your eyes and LEDs are much faster than the monitor.

What does the dealer or manufacturer say? Is this a problem for you, or just an annoyance?

Open up a light fixture and attach a multi-meter in parallel with the power leads for the fixture. Turn the light on, and watch the meter closely while someone else operates a faucet somewhere. What numbers are you seeing?