In my experience, it's a reflex.
In mine as well. I mostly teach OW courses but I teach students from the very beginning not to give an OK to confirm a sign their buddy is giving. I teach them to repeat the same sign back again.
Repeating the sign tells the person who initiated communication that the sign was received, understood and properly interpreted. An OK only tells you that they saw you wave your hands around.
For example, today I was with an OW student and we were ascending back to the shallows. At 5m I signed to do a safety stop. Here's how it went:
(after making eye contact)
me: level out
diver: level out
me: 3m safety stop
diver: 3min safety stop
me: swim in that direction
diver: swim in that direction
Pretty much no way to get confused right there. it's something that spilled over from my technical training into the way I teach OW.
What you often see, however, is this :
(no eye contact)
diver1 : 3 min safety stop
diver2: OK
diver1 starts swiming
diver 2 follows
You KNOW this is true.... all of you instructors out there know that this is what happens.
The potential for miscommunication in the second attempt is HUGE and miscommunication happens often. I would love to see PADI adopt the Diver0001 method as a standard but unfortunately most divers are not trained that way and end up giving OK signals as a conditioned response to anything someone waves at them, even if they didn't see it or didn't understand it (or thought they did when they didn't).
I'm not going to suggest that this is what happened in the Rob Stewart accident because I literally don't have enough information to make that determination. This post is only to respond to what Bill was saying and to suggest that with good training from the very beginning something of that nature can be avoided.
R..