The water heater switch—is it turning on the propane heating or the electric element (if you have one)?
If it's the electric element, the light is almost certainly just a 120V pilot light of some sort, and any old replacement would work fine—a lighted switch of sufficient ratings, or a separate pilot light or neon indicator.
If it's the propane side of things, the light (at least on mine) is a bi-color LED and is wired separately from the switch, as I recall. Most likely cause of the light not working would probably be a broken wire or other connection problem, not the LED itself.
Generator start switch is probably most easily ordered from the generator maker as a replacement part. Doubtless it would be possible to find a switch from other sources that would be compatible, but doing so would take a fair bit of searching, I think, particularly if you want one that's the same physical size. The generator manual may have the electrical configuration of the switch, which would be helpful for testing. Here also there's the possibility of a problem with the wiring between the generator and the swtich rather than a problem in the switch itself.