Start with checking for power at Romex into water heater . Inside on left as facing wh from outside. If good, then problem is burnt wire between t-stat and switch or between switch and element.
I have coated hundreds of roofs professionally. I use a product from Ry and Marine Technologies. Roof Magic. I lasts 10+ year (I have some roofs I did 17 years ago that are still good). Expensive, $50 gal stuff garbage.