I have been running straight water as coolant in my truck. As we have very few days in the single digits, I have had no trouble. However, today dropped to 7 degrees. I allowed my truck to warm up approx 15 minutes before leaving. As soon as I hit the road, the temp started to climb. Less than a mile down the road, the "Check Gage" light came on and I immediately returned home. If the temp gets above freezing will I be able to drive the truck?
The probable answer here is NO. Engines have freeze plugs in them which, when the block and coolant freezes, will pop out. This is a relief point so as not to damage the block (does not completely rule out block damage). When this happens and the coolant thaws, it will flow out of the block. This is the reason the temp went up and the check engine light came on. I would bet you also saw steam from under the hood as well. Freezing coolant can also damage the radiator. There is a reason it is called Anti-Freeze ... it's there for occasions such as these.
There are other reasons besides freezing why it is bad to run pure water as coolant. Water causes rust and corrosion in the block, which when it starts flaking can cause blockages in the heater and radiator cores. There are also anti-foaming agents in the anti-freeze which allows it to cool better. There are a bunch of other reasons as well, but I'm afraid you are probably hosed at this point. You have more than likely cause some engine damage. Hopefully the overheating has not cause further damage to the heads as well.
My suggestion to anyone out there is to NEVER run regular water as coolant. (There are additives such as water-wetter which work, but then the water is treated, so don't pertain to what I'm saying.)
EDIT: I was given great advice by moderator Bob Cross to add two pertinent links. Running with straight water as coolant and Using only coolant or mix of water for radiator, both of which are from this SE site.
I believe you may be lucky and this isn't permanent damage. If you have a garage, pull your truck into it and stick a nice electric element heater under it and wait a few hours. This is a more gentle method than what my grandfather used to get his tractor started on cold mornings: he literally made a fire under it.
Winter conditions require you to have 3 parts antifreeze to 1 part water in your coolant tank. High heat summers require the opposite: 1 part antifreeze to 3 parts water. Or 50/50 if you're too lazy to change it twice a year. Antifreeze does three very important things: