It works pretty well, especially if the pants are not worn through or have no holes. I can't speak for slacks one way or the other.
It is a pain to knot off the legs of wet blue jeans while treading water, but it can be done.
A 'drown-proof' float will also work if you can't master that, (tying the knots under water) but be careful and don't drop your pants! Shoes go first. Let them sink. You'll need to add air every so often, mine held up pretty well, and I only had to add air every 5 minutes or so.
If this guy was dealing with wind, the water may have been rough, too, and that will wear you out fast, especially if you don't know how to swim in it.
It was cold, and that will get you really fast.