Not sure if it effects the water resistance. It does however make it much quiter! I would imagine it might reduce it marginally, but wouldn't think it would be anything substantial. I use tyvek as ground sheets and i wash them a few times to make them more subtle and less noisy but have not noticed any ill effects(water leakage etc)
Frog Togs are made out of something very similar to Tyvek, well it probably is tyvek, but i think it may be a slightly thicker version? I don't have a frog tog suit to directly compare to any tyvek so cant help you there.