Can I install it myself? Thanks, Jill. You go girl, yes you can! BUT 240VAC can be dangerous. Make sure all power of off before attempting.
Home Depot and Lowe's carry them. Most HW heaters are 240VAC. Check to make sure. Need to look at wattage of the heater and make sure the switch can handle. With 240VAC, 2 hots and ground from box, 2 hots and ground through switch. If 120VAC, rare for house HW heater, 1 hot, 1 neutral and ground. Watch this.
https://www.youtube.com/watch?v=2tO4grzZ9iU
Check back if you need more help.