Most of this is what the boat crew is for. Obviously I assist in this issue, but it is not my primary role when DMing. It's incredibly annoying that this has become the role of the DM on most American charters.
Often times it is up to the DM to be sure the divers are in or out of the water at a certain time. Keep in mind we're diving the same boats and the same sites every day. We have a schedule to follow. Our employers expect that we make that schedule. This is especially true on a liveaboard when we're doing 5 dives a day. If you gave me the "one finger salute" after I told you we need to surface or to rejoin your group or buddy... you wouldn't be diving anymore.
You would never have to tell me to rejoin my buddy if I had one. When diving as buddies separation is not an option. I understand there is a schedule. If you set that as one hour I will be out in that or less. I don't need you to tell time for me. But if on that site I find the area around the mooring or anchor intertesting I will not be following someone to see a sea horse that may or may not be there. I know what interests me better than you do.
If there is an emergency there will or should be a recall signal. I will heed that immediately. I keep track of my own air pressure as well. If I am told I have an hour dive and a couple air hogs drain their tanks in one half that I'm not surfacing for them. If that's your boat's policy I will know that before hand and not be on it in the first place. I do not dive with anyone that treats me like a baby or like an incompetent diver or assumes I need to be told everything to do. I also try very hard to avoid boats that put DM's or Guides in the water. And when I have had occaision to use one that does it takes about 10 minutes before they allow me to do my own thing.