statement/reply? I always get an "ok" from my customers right before we descend, but in general, I don't use it much after that unless the situation warrants. As a new instructor, I probably "ok'ed" everybody about every 10 minutes. Now I can pretty much tell if they're ok by looking at them, so I let them look at fish or whatever instead of me all the time. When we have a deep dive planned, I'll tell the divers once we get to the "staging area" at about 18m/60', I'll give a "descend followed by ok" signal...having already explained to them that if ANYONE doesn't feel comfortable for whatever reason about going deep, let me know at that point & we'll keep the dive shallow. I've found it helpful to take a couple minutes to evaluate them there anyhow, before dropping down to 30m/100' +.
Anyway, regarding what I started with, I usually "hold" the ok sign for a while to make it a "question," but if it's a situation like a diver has a few bubbles leaking out from their inflator connection I'll do a quick "ok" to tell them "it's nothing to worry about." Some first stages (Sherwood, I think, for one) have a trickle of bubbles designed into them, for example, but divers using rental equipment often aren't aware of that.
I don't usually acknowledge other divers underwater (unless they signal ME first) except for other guides/instructors, most of whom I know....and in that case it would be something very subtle...a slight tip of the head/nod, or maybe extending my fingers (I usually have one hand holding the other's wrist during a dive) a bit in an abbreviated wave.
I try (within the parameters of keeping everyone safe & showing them the "good stuff) to follow what I heard about being a "good referee"...if people hardly notice them, that's what they want.