Galvanic reactions or electrolysis as I know it from the yachting industry are present any time two dis-similar metal are in electrical contact. In the case of a PC cooling loop that would be the fluid--which is usually conductive to some degree.
Yeah, I'm a dive boat captain so I'm well familiar with it too. I don't think the fluid can be the connector AND the electrolyte. You need a complete circuit. Current will flow from the cathode to the anode (aluminum to copper in this case) through the fluid and return via the electrical connection. The electrical connection is usually physical contact between the two metals such as when a bronze prop is mounted to a steel shaft. I suspect a lot of pc cooling installations inadvertently ground the physically-separated metals like the blocks and radiators when it could be easily prevented. If they are both grounded, we have a return path and electrolysis can occur.