When actually in the water with waders on, the water pressure forces the waders tightly to your body, leaving any places water can enter the waders very small in size. This, along with a wading belt keeps you pretty dry inside the waders. Just as long as you re-enter your yak quickly, you will stay dry. Stay in the water too long, and they will start to seep, then fill. The Jim Sammons video mentioned above explains/shows it best.
