A good book to have is Jeff Bennett's "Guide to the Whitewater Rivers of Washington." If the float you're thinking of doing is in it, you might want to read about it from a whitewater boater's perspective. Then, you might want to consider your skills, the river level and weather conditions (like, you probably don't want to flip your boat on the Sauk in February).
As an example, the Sauk between the Whitechuck R. and Darrington WAS a serious class IV whitewater run. Not really a good fishing float, especially in winter. Definitely not canoe water unless you're a whitewater pro, literally. You'd be better off wearing a wetsuit and a helmet up there, as opposed to fishing garb.
Below Darrington it WAS ok, with a couple of class II spots and a few obstacles to miss. Now, after the floods, it's pretty much unknown, with the likelihood being that it's going to be much more technical than it was before.
Misc. River Dangers: As Bob said, fresh sweepers plop into the rivers at random. Keep you eyes open and on the water ahead. Watch for eddies to bail into. Scout anything that looks challenging. Etc.
Sweepers have a companion danger known as "Strainers" - these are branches, etc. in the water that act as a sieve. The water pressure will pin you against them and you'll be toast. Many whitewater boaters have been killed by strainers. Often, if there are lots of trees/brush in the water it's safest not to swim for shore and to instead float mid-stream (PFD) and wait for your buddy to rescue you - you are floating with a buddy, right? Better yet, a buddy with a throw-bag to help reel you in.
Lastly, (nobody does this) wear your stupid PFD while you're floating. Then, take it off when you get out to fish a pool. The Yakima canyon in summer is one thing, but Wetside rivers in winter are a whole different story. Consider what it'd mean to capsize your boat, given the conditions. Also, on the plus side, a PFD adds insulation that helps keep you warmer anyways. If you're serious about floating, go buy a kayaker's PFD. They're constructed for maximum arm mobility.
The Sauk is/was a relatively dangerous river. The Skagit was far more mellow. Try that first. It's where the Sauk fish come from anyways.
Cheers!
DP:thumb