I stumbled across a great article by Garrett on water-cooling turbos that addreses this issue specifically on page 7.
http://www.turbobygarrett.com/turbobygarrett/images/tech_center/Garrett_White_Paper_01_Water_Cooling.pdf
I recently read about someone attempting to adapt a water-cooled turbo for an air-cooled motorcycle engine. His plan was to run oil through the water jacket to provide cooling for the turbo.
Is this a realistic possibility? Would it be better to just use a normal "oil-cooled" turbo?