Interesting read...
I was taught that Thumbs up means ascend until the "level off" or "stay at this depth" sign was given, if that sign was not given, that means your slowly ascending to the surface...
If you want to indicate "let's go up and do our saftey stop" I was taught to use the right hand to give the thumbs up, and then take your left hand open palm down and then move the right hand so that your thumb hits the center of your open palm (as if hitting an overhead).
Too bad I don't have my digital cam handy, I could take some picks of an U/W conversation I had with my DM on Maui,, It went something like this
Me to DM: I'm at turn pressure, I have (whatever it was I had, let's say 1000psi for this example)
DM to Me: OK, do you see the boat?
Me to DM: OK, the boat is that way, my buddy and I are heading for saftey stop
DM to ME: OK
Me to DM: OK
Me to Buddy: Let's go do our stop, follow me, OK
Buddy to Me: OK
Let's see now, Turn Pressure sign was breifed before the dive, I think most people usually use this sign for "1/2 tank" but on this dive it was to mean turn. Take left hand open palm up and place your right hand (open) with the pinky edge across the middle of your open left palm, as if cutting your left hand in half (pinky to thumb) with your right.
Boat: Cup both hands into a V shape
Do you see? / Look : point to your eyes with forefinger and middle finger
That way: Point!
Hope this makes sense...
There are also a few signs you can use for various wildlife...
Shark : Make a shark fin with your hand on your forehead
Turtle: Make a fist with both hands and place your right hand on top of your left, extend your thumbs and then rotate your thumbs in a circle
And of course the diver that kicked me in the head got the "your #1 in my book" sign (I won't describe that one)