Electrical/Plumbing Question

I’m in the planning stages of adding a new water heater. I want to add a single element 1500 Watt point of use 12 gallon or 19 gallon water heater to my spare bath. In order to avoid running a new dedicated circuit, I was thinking about connecting the water heater to a 20 amp circuit that only powers the GFCI outlet in the spare bath. I know I can't power both the GFCI and the water heater, so I'd like to add a Leviton 1287 Single Pole Double Throw switch to allow me to select whether to power the GFCI circuit or the water heater. The 1287 is rated [email protected], so it should carry the water heater just fine.

The idea is use this setup only when the power is out and I have to switch to a generator. I plan to cut the line in the attic, use a junction box, and run a line down the wall into a closet where the water heater will be located and connect to the 1287 switch. From there, I'll run a line to the water heater and another back up into the junction box to power the GFCI. I can also go directly through the floor and connect the water heater inline with the hot water supply line. If the plan works, when the power goes off for an extended period of time, I can go into the closet, flip the switch from GFCI to Water Heater, and have enough hot water for a short shower.

To my mind, this should work, but I'm no expert. Any electrical or plumbing experts want to weigh in on this?