It is difficult to give this a name. A "circling line" is attached to the "descent line" and allows the diver to dive in a circle around the descent line (which is usually attached to the bottom of the body of water or somehow fixed in it's position - see: http://www.tpub.com/content/diving/NavyDiving/css/Dive_Man_C...

For general info and words used in "cave diving" (where descent and circling lines are used very often) see: http://www.lochstein.de/sprache/dictiona.htm

:)

The "descent line" could be translated as "Abtauchleine".