No, I can't really recommend a dielectric coolant.
Alcohols: not dielectric, but less conductive than water. Lower specific heat capacity than water. Higher viscosity except for methanol which is less viscous, but I would guess that this would not be enough to offset the 39% lower specific heat capacity and 58% lower thermal conductivity. Flammable. Toxic, especially methanol (which is also the most volatile, i.e. it has a low boiling temperature and evaporates very readily). Ethan-1,2-diol (ethylene glycol, the main component of antifreeze) is very viscous, and less volatile than water.
they are flammable and have less than half the specific heat capacity of water. Butane (C4H10) is a gas at room temperature. Pentane (C5H12) has low viscosity and boils somewhere in the 40s C (I don't remember exactly). Hexane boils at 67-68 but is badly toxic. As you get into heavier things, oils etc, the volatility decreases (and therefore so does the fire risk) but the viscosity increases, so you have trouble pumping the stuff. Also, oil is unpleasant stuff to clean up if it leaks. Hydrocarbons may dissolve or at least soften some kinds of tubing.
Chlorinated aliphatic compounds:
1,1,1-Trichloroeethane is just liquid but evaporates very readily; I'd have to look up the boiling point. Really rather bad for you. Dissolves many kinds of plastics, lacquers, paints, tubing etc. Not much of a fire risk.
Dichloromethane is similar but only a bit bad for you. Both of those have low viscosity, but I don't know the numbers.
Chlorinated aromatic compounds:
PCBs (polychlorinated biphenyls) have been used successfully on a large scale for cooling large transformers. However they are badly toxic and I don't think they're legal now. Don't do it. (I expect they would dissolve many plastics too.)
specific heat capacity much greater than any of the alternatives. Low viscosity. Most people who've had spills have found that their equipment works after drying out.
Oh, for some numbers for water and some alcohols you might like to look at this thread from the overclockers.com forums:
http://forum.oc-forums.com/vb/showthrea ... adid=39039
Bear in mind that high thermal conductivity and high specific heat capacity are both good things. (Specific heat capacity (thermal capacity) is the energy absorbed per unit temperature-rise per unit mass.)
A word of warning though: in that forum thread, the person who gathered the data seems to think that a high ratio of conductivity to capacity is good, i.e. low capacity is good. This is plain wrong. In fact multiplying capacity by conductivity might give a useful measure of the performance of a coolant (though viscosity must also be considered).
Off-topic note on links:
The link above as copied from my browser had something like "s=1235manydigits6789&" between the "?" and the "threadid" but as you can see it works fine with that chunk (the session id) missing. This way, this thread doesn't get wider than I like to have my browser window.