This hike has been on our bucket list for a long time. We go down to Zion National Park almost every year, but this is the first time we had the logistics in place (permits and daylight hours) to walk the narrows. Its a fairly easy walk - around 16 miles from Chamberlain's Ranch to Temple of Sinawava. You are in the water for roughly 14 of the 16 miles, although with normal water levels (40 cfs) at this time of the year, you are never in much deeper than your knees.
Its truly a spectacular walk and we were just in awe almost the entire time. Our photos don't come close to doing the narrows justice.
A couple tips if you are planning to do this...
1. Use two trekking poles. Then you can look around constantly and not have to worry about your footing and slipping.
2. Neoprene socks are a MUST have! Rent them from any outfitter in Springdale.
3. Prepare for a range of temperatures. We were COLD in the morning - wearing gloves and hats until the afternoon... and the daytime high temperature ended up being ~90 degrees. It was hot and you didn't want to get out of the water by the end.