In the UK/Europe the Nest 3rd generation supports an on/off function for hot water. I have made a pull request to python-nest to expose some more of the properties for this functionality and am looking into sending commands to it using them (I think you can, but just needs a little more testing).
Meanwhile I was wondering how this could be set up within Home Assistant (at least to start with just to report the status) - should it be part of the current Nest climate component (using a check for
has_hot_water_control), or should there be a new Nest climate hot-water component, considering it would essentially have it’s own ON/OFF state? (I’m still getting my head around the source code!).