[m_mueller]

Your idea is what's being done at the largest computing center in Switzerland, the CSCS. They pump water from the relatively deep Lake Lugano [1]. Europe's currently fastest supercomputer (world No. 7) is hosted there.

[1] http://www.cscs.ch/cscs/an_innovative_centre/cooling_system/...

[2] http://www.top500.org/lists/2015/11/