I hiked the Narrows two years ago around Halloween - you are going to love it. My two cents as follows, forgive me if I am repeating advice you're already received. . .
Check the water flow rate of the Virgin River and weather forecast before going. References to flash flooding in that slot canyon are no joke and happen very, very fast and can be from storms you never see far up river. Rent the proper canyoneering river shoes and dry pants from one of the local shops; I used the one down by the visitor center, but there are others in town. That gear made the hike so much better than it would have been in other footwear and I stayed warm. Yes, you'll see folks in all kinds of shoes (and even no shoes!) but ignore it. The river bottom is often like walking on slippery bowling balls with a current pushing you. You'll want a big walking stick to brace you against the current and for balance, which unfortunately is another thing to carry. While basically flat since you're hiking in the river (and along sand / rock benches), it can be a tough hike given the conditions, so be in shape for it, as it's about 10 miles r/t if you go as far as you can go on a day hike. I wore sandals on the shuttle bus and on the paved trail until I got to the entry point of the river, then put on the canyoneering shoes. Reverse plan on the return for walking comfort. Think hard about how much camera gear you want to carry all those wonderful miles; I stuck with a single 24mm prime and had no regrets. Wider might have been fun, but the distortion in the narrow canyon might be too dramatic. A couple times I wished I had something with more reach for distant fall foliage etc, but again, no regrets. Your tripod needs to be solid since most of the time it will be in the river and since it's so dark in many sections, a tripod is a must for me for the slower shutter speeds and stability . I bracketed some shots with enormous dynamic range and used a polarizer and no ND's. I carried a hiking backpack (lots of water, snacks, lunch, first aid, clothing layers, headlamp, etc) and a waterproof sling bag for the camera and lens, tripod, and sturdy wood hiking stick. That's a lot to juggle, but I decided I wanted to make sure any accidental dunk in the water wouldn't damage my gear and it worked out fine. Sometimes I put the camera away, sometimes I was confident enough to leave it on my Peak clip on my shoulder strap. The rangers advise taking enough provisions "just in case" you have to overnight in the canyon; that might seem like overkill, but I can see the justification. No cell and GPS is very sketchy due to the high canyon walls. Get on the very first shuttle bus in the dark to get out in front of the crowds, as the speedy hikers will eventually catch up with you. Solitude in the canyon is. . . just amazing. Have a great time, take extra batteries and memory cards, and have fun!
