You need to be well-prepared for canyoneering (definition) trips to avoid disaster. You need the proper equipment and training. Some of the trips will require rock climbing, rappelling, and swimming through ice cold water with all your hiking gear. It's better to scare away a few people who could have done a canyoneering trip than have a disaster because of one person who shouldn't be there.
Since canyoneering can be so dangerous, and everyone's safety is very dependent on every participant, canyoneering trips are usually only open to a limited number of experienced canyoneers with whom we've already done trips. The whole group must be comfortable with the abilities, personalities, and judgement of the entire group.
All canyoneering trips are totally dependent upon the weather, both during the trip and in the recent past anywhere that could possibly drain into the canyon, since in slot canyons flash floods can kill people very easily.
On my first canyoneering adventure, The Black Hole in 1987, our entire group spent the night in the bottom of the canyon in wet clothes, with no food or camping supplies, because we couldn't get out, and the only things we had managed to keep dry were the cameras. The bottom of a slot canyon is a very dangerous place to spend the night. It was the first slot canyon for all of us, none of us were climbers then, and we very foolishly thought we could do the canyon somewhat comparably to our guide book author, Michael Kelsey.
On another trip in the same canyon, part of our group spent the night in the canyon because we had one person with us who couldn't deal with the heights and exposure.
On one trip to Buckskin Gulch, the half of the group doing the "short" hike didn't make it back to camp till the next day, so the rest of us were really worried. They had just been hiking too slow. (They were taking lots of photographs. Slot canyons are beautiful!) On another trip, one person didn't bring enough water and had to "borrow" so much that we all ended up really hurting. On yet another trip, what little of the sky we could see started looking worrisome, so we had to hurry to one of the very few places where it is even possible to get out of the canyon, climb out (not trivial), and then find our way back to camp, cross country, from some place other than we had planned.
A day after we hiked in West Clear Creek, a group of three (that we had started out with the previous day) was hit by a flash flood. One hiker, Wayne, lost all his gear and got separated from his group. One of his friends became sick, literally, with the fear that Wayne was dead. (Wayne was fine.) By the way, some other people from the first group were in another part of West Clear Creek the same day and saw no flash flood!
Here's a link to a cautionary tale about canyoneering:
http://www.canyoneeringusa.com/utah/story.htm.
Here's a link to a near-drowning report by two members
of our canyoneering group:
http://members.cox.net/kathy.m.sharp/reports/accident_2005_04.html.
You can read other canyoneering accounts, including one
about the canyoneer who cut off his own hand and another
about the two canyoneering deaths in April of 2005, at
this link: http://climb-utah.com/tales.htm.
And here's a link for a canyoneering organization: http://www.canyoneering.net/.
May and June is usually the best time of year to go on these trips to avoid flash floods; avoid boulders dislodging from way above you because of soggy, vertical canyon walls; and not freeze in the cold water.
Due to the climbing, setting rappel ropes, hunting for routes, swimming, ascending ropes, etc., an average canyoneering speed is 1 mph, about half the average speed for hiking. In one canyon we spent all day to cover only one mile.
Check out the canyoneering supply list at the Checklists link on this page or http://members.cox.net/kathy.m.sharp/checklists/.