Hmm, interresting idea. Maybe I can try doing that on my off days. Since the grip muscles are small, they recover fast, shouldn't be a problem.
And Trevor, do you mean squeezing the bottle itself. Did you use your thumb? Or did you just use the four other fingers? Or did you do like a pinch grip, pinching the bottle between your thumb and fingers?
All three actually, mainly just full on squeezing but I did some work in those grips as well.
also, make sure the water bottle is made of thin plastic. You want to be squeezing the air. Not against some hard plastic.
The thickness of the bottle can also make it a lot more difficult. Try to get one you can wrap your fingers/thumb fully around to start with. The more "girth" the bottle has. I would assume the less muscles worked (never studied the hands skeletal muscle system much lol).
Oh, And I never did any type of RSS (rep-set-scheme). I just kinda squeezed until my hand got tired lol. Though, now days I could do it hours on end. So I work with Isometric holds, fastest times to get to a certain number of full squeezes, and have set up a RSS nowadays.