Hello to all!

Well, yesterday I pulled a bonehead move.
I was driving along the interstate when I saw that the coolant light on the dash was on very faintly. I pulled over at a rest area, pulled out my coolant from the trunk (about a quart left in the jug), and like an idiot (using a towel 'cause I knew it was hot) unscrewed the coolant reservior cap and proceeded to boil over ALOT of coolant. Luckely, I didn't burn myself.
I went into the rest area and filled the jug with one quart of coolant with tap water and poured it into the reservior. I took over a gallon and when I got home I noticed that it was 'boiling' in the reservior due to overfill. Since the incident, I've driven about 40 miles and the temp guage never got above approx. 90 degrees.

Questions:
The bulk of the coolant in the car is tap water. Is this safe?
Is this the perfect time for a coolant flush? I've never performed this service in 150K miles.

Nick