I answered less than 1% because I have only ever had one dive where I lost my buddies and couldn't find them. We were shallow, 35-40 foot, but with a 3-4 knot current across a 300 year old wooden shipwreck we were mapping. There were three of us in my team, and a camera crew filming us work, and two other teams working elsewhere on the wreck, visibility was poor at 5 or 6 foot (severely hampered the filming
).
The film crew wanted a repeat of a shot, we stopped and did it, then I looked up for my buddies and they were nowhere to be seen and I did not know which direction they had gone. Several people were in FFM with comms so we checked in with the boat and knew everyone was safe, I couldn't see any of the other teams so I just stuck with the camera crew and surfaced with them, I never did find my dive team until we surfaced.
But - if you want to know how many times I have lost sight of my buddy, then the answer is different, at certain times of the year quite often.
Fairly often I do very low visibility dives, by which I mean I have a hand on my buddies tank and still can't see him. In these conditions it is quite common to lose sight and contact. It happened three times on a dive last weekend. But we plan for it, one diver swims half a bodies length ahead, the second diver keeps physical contact with the tank or hip of the front diver, if the front diver feels loss of contact they stop, the following diver then catches up/searches on the appropriate side until contact is regained.
In all three cases last weekend contact was re-established within probably 15-20 seconds or so, but it would have been a nightmare if both divers were swimming around looking, we would probably have missed each other.
If we don't find each other within 2 minutes we surface and rejoin on top. We could of course use a buddy line, and have done at times, but generally we don't like the restraint of one of them. - Phil.