question

Isn't it simplest to just add a receptacle in the shorepower line before the Multiplus and plug the water heater into that? When you aren't on shore power it won't heat water, when you are it will.

I thought about this but then the cerbo GX display will not accurately show how much power is drawn from the shore power if the energy utilized by the water heater doesn't flow through the inverter-charger

On our sailboat, we gave the hot water heater its own breaker (it should have its own anyway). So, sitting here at anchor, we have the tank breakered off to preserve battery. However, given our intention to motor a long way tomorrow, we’ll probably turn it on to do dishes tonight.

Next engine will have a freshwater coolant loop, so it can heat the hot water tank while we motor directly.

Back to the original question, is there a way to program the aux relay so it's on only when inverting (AC-in off)?

You can use the programmable relay assistant to switch based on AC-In available or unavailable, which is logically the same as a model with an AC-Out2.

Wire the coil of a contactor across the mains input?