I have never been on a boat that had more than 16 divers. This may sound like a lot but the boats are very roomy. Generally, most of the boats went out with fewer people. A full boat may have up to 16 divers, 2 divemasters and a boat captain who stays on the boat. I have also gone out on boats that have only had 5-6 divers. For the first guided dive, they would divide you into 2 groups of 8, one per dive-master. The second group usually goes in about 5 minutes after the first group so that you stay completely separate. They guide the first dive down to 100' and keep you close together. After you shallow up, we just separate into buddy teams and do our own thing. We went up when our computers said to. Now, of course, you don't want to be a pain and keep the rest of the boat waiting if you are going to stay down 15 minutes longer than everyone else. That said, there is no one in the water to enforce their tables. My wife and I would typically be the last ones on the boat and they never made a point about us coming up late.