Indeed, - I just looked, and I couldn't find that option either...
I am not using smart watch myself, so, I don't use that option.
I know this option had been discussed here before, and users have successfully used it. Either it has been removed from the recent version of Aquamail, or some type of app/extension that handles smart watches has to be present on the device for this option to be displayed.
Sorry... you may need to wait for response from Aquamail developer or support.