[kyleb]

I added a solid state relay from Automation Direct ( #AD-SSR625-AC-280A )

25 amp capacity, 90 - 280 vac

If you get a 120v rated switch and use this relay, you won't need to pull any new wires.

I think this is how I wired mine . . . .
The hot supplying the switch also supplies power to the relay (terminal 1). The wire from the switch that used to go the the heater now goes to the coil side of the relay (terminal 4).
Terminal 2 now supplies power to the water heater. Connect terminal 3 to ground.