It takes 2 hands to clap. I’m pretty sure SmartThings are proactively seeking out to integrate with Rheem, it’s really up to Rheem if they are willing to do their part.
The use case for water heater isn’t a lot. However, I’ve wrote this device type for water heater only if you’re interested. It’s still a little buggy. I haven’t have the time to write the SmartApp to change the temperature when away.