Clan Sleekit

Many of the byways of the Under-Empire are linked by subterranean rivers and seas. While most Skaven avoid these regions for the dark things they are said to contain, Clan Sleekit sees these routes as a commodity. Expert boatmen and navigators, the Skaven of Clan Sleekit are said to have explored the dim reaches of the world below, and harbour terrible secrets about what other things lurk in the sunless world.

Source

 * Warhammer Fantasy RPG 2nd ED -- Children of the Horned Rat pg. 50