If you are looking to hike from the top to the confluence and back, I wouldn't recommend that. Confluence is getting close to the 1/2 way mark and you would be better off hiking the entire canyon. Some folks do the whole 16 miles in a day hike, but thats a long hike, and you probably want lower water for that. If you want to hike from the mouth of the canyon upstream, thats a good day hike. Most day hikers won't make it to the confluece with deep creek, but you can get into the deepest part of the zion narrows (wall street) and see some really cool sights. I'd hike up from the bottom, and turn around as timing / weather dictates. Not sure what the current flows are like for hiking. You would be in the water a lot and would want good drygear.
Google zion adventure company if you want some more info on zion hiking and canyoneering. Cool info there. They also rent 5.10 canyoneers which are the most badass shoes to be hiking in a riverbed in my opinion. I use the canyoneers for boating now after getting a taste of how good the traction is on wet rock when I did a top to bottom hike of the narrows in the fall. They can also give you some beta on flows and how it impacts hiking.
Do some digging on some of the canyoneering sites too. There are some really cool slots in some of the side canyons. If the narrows has too much water, there are many other cool hiking options. If I was heading to zion, I'd bring my drysuit and my best water shoes and go hiking through the slots.
Here is a link to some hiking options on the park website. They quote 120cfs as the max for hiking permits, but apparently a day hike from the bottom doesn't require a permit unless you go above big springs.