I think drills are appropriate during camp. You want people to feel prepared in case of an emergency, and also you want them to be able to stay calm. You want them to be able to go through the steps automatically. the only way to achieve this is to practice.
A good way to prepare the campers (because you don't want them flipping out) is to let them know that just the way they have fire drills at school, sometimes we do drills like that at camp...be it a fire drill, missing campers, distressed swimmer, etc. If they know ahead of time of the possibility, then when the time comes they probably will be able to calmly follow the instructions that are given them (and, they will mirror the staff...if the staff is calm yet serious, they will be OK. If hte staff is panicked and out of control, it will be a bad situation).
At my camp, we usually do a distressed swimmer drill about once a week, with campers in the pool. It takes less than 5 minutes to go through the process (of course, since its a drill, we leave out the "911" step).
We've also done fire drills. The camp director was usually kind enough to look at everyone's unit schedule to find a time when no units were scheduled for horse time, swimming, or showers. So we weren't pulling people off of horses or they weren't going throught the drill wrapped up in a towel. usually right before a cookout dinner seemed to be a good time for a fire drill. Or within 5 minutes of dismissing from a meal (they haven't gone too far yet).
I remember one time there was a missing camper drill. It didn't turn out so good, because the staff member who realized the girls were missing did not want to report it to the camp director, and so if it had been an actual missing camper, valuable time would have been lost. Also, when that staff member realized it was a drill and that the campers were with the camp director, she went ballistic. although the funny thing is that when the camp director tried to "lure" one girl away with the promise of a popsicle, the girl said she couldn't go without her buddy (!) so it turned into 2 missing campers (she brought her buddy along). So at least we had drilled into her head the importance of having a buddy, but apparently we didn't do such a good job of teaching her to stay with her group.
