Oh yeah... The diver that was right here with us on the boat when we came out... Joe. He's right he... Joe? Joe!? JOE!
Oh ****... Someone go get him!
Just kidding...
I've only had that happen to me once. Luckily, my buddy came up and saw that I was not going to make it. He got in the boat and came and got me.
To me, my alternative was to swim to shore, doff my gear (make it visible from the water) and hike a mile or two upriver and get back in the water... Without gear, but WITH my fins. I'd have been pretty ticked, but that's the drill. Buddies who dive with me know that if one of us misses the boat, the plan is to swim to shore, leave gear out so that it can be seen, then do the hike. From there, it's a matter of deduction to find each other again... That is, if the plan is not to surface 2 minutes after losing your buddy anyway.
I think the real right answer is to know where your buddy is at all times - AND to have someone on the boat that can come and get you (has to be able to drive the boat).
...But since dive locations for fossils are so coveted and secretive, and because we can never find anyone just willing to sit on a boat all day, we don't always have that opportunity.
Anyway, that's how we do it. I don't know that I'd recommend doing it that way - it's a much better idea to always have someone on the boat. Having someone on the boat also works in all situations, which means that it's practiced and practical and consistent, making the dives routine and therefore easier.
'Guess I'm an "Around Here" boater.
Doesn't mean that's the RIGHT way to do it, though... That's just what we choose to do.
Re: "Thumbs up" scenario... I'm gonna agree with Tim on this one - I think I'd double-check what my buddy was trying to say. That said, to me, "thumbs up" means "end the dive" - right now, with no argument. I will never be upset or otherwise give someone a hard time about ending a dive, even if I don't understand why they're ending it.
I've had situations like that where, on our ascent, I was thinking, "What the heck?" When I got topside, I found out that they'd suffered deep jellyfish stings all over their face and were in quite a bit of pain. They had no way of communicating that to me at the time.
Glad I didn't argue... I'd have felt like a real butthead.