Oh you got a lot of great questions there.
First off, have you played around with our business estimate calculators
They really get you to start thinking about how your expenses effect your hourly rate.
Play around with them and see if they help you come up with a better understanding how much you should charge.
What you want to do here with the watering is labor intensive. I can see it taking an hour to do this.
if it were me, I would either bill her an hour to perform the service or set up a sprinkler with an automated timer and have it water the lawn daily, weekly or whatever.
Then I would bill just to set it up and rent the equipment.
If you don't need to be there and you don't want to bill the full rate to be there, than why not make it easier on yourself and set up a sprinkler.
What's your thoughts on that?