Not sure I would go there. I'd hate to turn your water heater into a impromptu steam boiler. A PID that can be set well above the boiling point of water would just be scary to me controlling a water heater, you aren't gonna find one not made in china, and you are one malfunction away from disaster. I know water heaters have multiple safeties to avoid this, but am not familiar enough with them to say it is safe.
All that said, if you wanted to do it you probably could the same way I mentioned for the oven. I'd imagine you could control pretty much any gas appliance that way.