Ill take a shot at this.... The river is between inches deep, and at least 40 feet in a few holes, probably deeper.
You could snorkel most of the way. Several of the rapids are only inches deep and you would grind your face off unless you got out, I consider this part of the adventure. I really wouldn't think snorkeling top to bottom would be much fun. It would probably get you messed with and maybe get you into a fight somehow during busy tubing times.
There are a lot of hazzard in the river. Junk, ropes, ass wholes tubing, and currents. You can get f'ed up and die.
Also Its fun.
Water speed varies alot depending on water depth and width. Float times vary alot depending on water flow. Pretty much nothing is constant Id guess 1000-2000 cfs flow rate and a float time on a tube from 2-4 hours.
Water comes from the bottom of Saguaro and is usually around 65-70 degress where you put in and it warms a little I think as it goes down stream. Last July downstream of the bridge I saw 72-74.
Visibility is not much. I've logged between 3 and 10 feet.
Before I thought about drifting the whole thing, I would go to salt river recreation with a cooler full of beverages, and a few friends, and tube it.
Then I'd probably do a few shore dives of some of the deeper areas and see what its all about.
THEN i'd make a decision about drifting the whole thing.
On second thought, forget everything I said. Here's the new plan...... It would be a fantastic idea to dive top to bottom. I recommend it to everyone. Nothing could possibly go wrong. Everyone tubing the river is sober and repectful and Im sure they would be happy to assist you however they can. The water is bathwater warm, and vis often exceeds 100 feet.
Thats pretty much all I have.
Any other questions? I'll try to help.
Jeff