I've done both. Lately when we leave the boat unattended we have a surface marker buoy that has a pelican case attached to it with a sat phone/message device, EPIRB, and flashing light. But I've more of a mind now to simply have enough people on board to be able to stagger our divers so that there's always someone up top that keeps the boat in idle and circles the marker buoy waiting for the divers to come up - this way I can come to them and get them, saving them from a long swim back, and they can show me ideal spots where good reef/structure is and next time we'll hit the bullseye on where to drop.