Assalamu aliakum, I remember Mufti Husain saying once that the material needs to be water proof and not the socks itself (as brother Ansari mentioned). Mufti Husain also said that the material has to be waterproof to the extent that if water is poured from a bottle onto the material it will not leak - not if it is dipped in water (too much pressure) nor while making masah (too little pressure).