My buddy Alex Tang (AKA: _Tango) has been running an Enertrac hub motor for quite some time now, and has decided to put in a water cooling system. Here’s his video on how that goes together. (This is with an upgraded Enertrac motor with water cooling included. More information here: Enertrac product pages.)
To follow his updates, you should be able to use this link on his blog: Watercooling!