Cancun Mark mentioned that guides will often spend the last 10 minutes (or longer, in my case) of the dive at around 15', basically doing a safety stop on the way back to the boat. If the reef contour allows this, I always do it....and even in deeper areas, I'll often swim back to the boat far above the bottom, just to "take care" of the safety stop on the way back. It's funny, though....sometimes we'll spend, say, the last 20 minutes back to the boat at 10~20', but when we get to the boat, some of the divers make a beeline right to the anchor rope, to "do" their 3-minute safety stop, not realizing they've already DONE at 20-minute safety stop. If I remember, I'll tell them in advance that on this particular dive, we'll have that already done by the time we get to the boat.....sometimes I think new divers think "safety stop = hold a rope." (I've seen "crowded" anchor ropes/mooring lines, where 20 or more divers are doing a safety stop at the same time....but because some of them don't have adequate buoyancy skills to do one w/out the rope, these divers are spread out from about 5m/15' all the way down to 15m/50' ! (hope they get in some sort of slow ascent/safety stop as they creep up the rope.)