Most likley just water, as it did not seem to have any adverse effects on Scotty once he was freed from it and able to breath again.
^^ Water is actually a poor refridgerant and most likely would not be able to deal with the kinds of temperatures and pressures of a warp engine.
In ST:TNG, Geordi is seen evacuating engineering when there is a coolant leak suggesting that the coolant is hazardous, possibly on more than one occasion (episode citation needed)
In ST:FC, the coolant shown for the ENT-E's warp core is some kind of chemical that is highly corrosive to organic material.
DarthLucifer 19:34, August 29, 2011 (UTC)
The "warp coolant" they flooded Engineering with in Star Trek: First Contact was called "plasma" by Data, and that it would liquefy human flesh on contact ProfessorTrek 22:21, August 7, 2011 (UTC)
There's a semi-liquid coolant and also gas coolant. Enterprise-D engines used a gas coolant, and this leaked out twice calling for engineering to be evacuated. When it leaked out the second time it may of caused/accelerated the warp core breach that destroyed the Enterprise-D stardrive section. Enterprise-E used a semi-liquid coolant, mentioned above.