Normal tap water, as well as bottled water, has more than adequate
conductivity to  support electrolysis even when mixed in a 50/50 solution.
The interconnection between two dissimilar metals will lead to corrosion. The
key question is the rate. The electron transfer rate (directly related to the
mass transfer rate is related to the metal surface potential, the
concentration of the solublized ion in solution (Cu2+, Al2+, etc), the
voltage drop across the connector (coolant circuit) and the electromotive
force difference between the two parent metals. The best way to minimize this
transport is to modify the surface potential and minimize the appropriate ion
concentration. The additive packages in commercial antifreeze products are
designed to do just that (along with buffering and lubrication, and ...).
They use surfactants which bind to the exposed metal surface rendering it an
exposed semimetal, semiconductor or insulator (take your poison based on your
point of view). The net result with surfactants is that the surface is
complexed and the equilibrium charge or mass transfer is lowered by factors
of 10*3 to 10*5.

A similar chemistry applies to the ion concentration in solution. Here,
clathrates or complexing agents are used to bind up the soluble ions.
Unfortunately, these compounds are hydrolyzed (react with water) and their
concentration falls off with time. At a critical concentration, no more
protection exists.

These rates and effective reactions are also a strong function of temperature.

The summary, Copper in a cooling circuit with Aluminum and steel will
corrode. The rate could be negligible (higher probability of personal meteor
strike than damaging corrosion of Cu component in 5 years or less).
Alternative is thick film hydrophobic film as I mentioned in an earlier post.