Pete, I think the word you are searching for is intertia. Yes the water in the waders will be neutrally buoyant but will act as if it were not when it comes to moving it. It will not drag you down, but makes moving your legs to swim almost impossible. Accidently testing this law of physics almost cost me my life many years ago when I got washed in and my hip boots flooded.
"Here fishy fishy."