Water Heater has 2 4,000 watt elements, upper and lower.

AFAIK dual elements are energized one at a time.

If powering with 120 VAC, the power will be about 1,000 Watta.

With 1KW into 54 Gallons of water, there is not a high probability of
boiling the tank.

Why wouldn't a pigtail 110 VAC receptacle wired across the lower
element/thermostat be sufficient as a connect point if the main breaker
and the WH breaker are both in the OFF position?