I know the OP is asking about an emergency situation, but just so someone new doesn't mis-interpret the answers above:

Don't run water-only, even in Arizona. In addition to its anti-freezing properties, coolant also contains lubricants and anti-corrosion additives to protect the entire system. In really hot climates, it's acceptable to lower the concentrataion to 30/70, but I wouldn't go any lower.

Edited by RF_Overlord (05/17/13 09:50 PM)
'03 Mercury Marauder (The Blackbird)
Trilogy Motorsports supercharged #61
Too many mods to list / Driveway Queen

'04 Grand Marquis LS Limited Edition (Spruce Green)
Daily Driver (Thanks, Adam!)

'00 Grand Marquis GS (Silver)
Rotted frame and broken transmission crossmember
Being resurrected...slowly.