Originally Posted by MB2timer

Anytime your coolant is low, your first suspicion should be a leak somewhere in the cooling system.
streborx had a great reply, when he talked about topping off with a 50/50 mixture. In lieu of that, if you find you are topping off on some regular basis, alternate between distilled water one time, and the recommended anti freeze the next.
One reply talked about tap water, and although not ideal, if you are trying to limp to the repair shop, it will do in a pinch, especially if your cooling system is going to get flushed and filled when you get there.
If I had to guess, the most likely cause of a slow leak like you are experiencing, is a water pump seal. Regardless, coolant leaks don’t fix themselves, and generally, eventually get worse.
There are some other interesting(to me) aspects to anti freeze combinations, but I will not bore anyone, unless someone begs me.

Originally Posted by W205C43PFL

Perhaps if it was (hard) tap water, it wouldn't be good right?

Originally Posted by MB2timer

I am kind of referring to the situation of a 1 time use. As in stranded. Even urine in that case could limp you home, especially if it was someplace like Death Valley.