That fatigue is related to hydration doesn't seem strange to me. So then that you can't heal it without access to water seems quite natural. One could argue that you cannot heal any of the traits without water.
Either way, the emphasis is on the REST and this makes it easier to narrate. The resource mechanics just forces a focus on survival gameplay, which seems like a good fit for the setting.
I see a bigger problem with how deadly it is to be starving. You die after just a few days when going without food. For water that would be fine. For food I'd expect to live quite a bit longer :)