Eldar do not swim. They disdain the water around them so that it transports where they were meant to go.
I'm going to play the straight man. Swimming seems highly unnecessary for the relatively regimented lives of the Eldar. I don't even see Dark Eldar swimming, so much as lounging in hip high stuff. Also, they wear armor and that would make them sink, and unless they are idiots who don't wear their helmets I MEAN TRUE HEROES WHO EXPOSE THEIR HEAD TO THE ENEMY/ATMOSPHERE then they are wearing closed system suits anyway, meaning swimming is again, pretty much useless. Of course, most Imperial Guardsman probably cannot swim either.