I also use the container that allows you to pre-measure your powdered formula. Some of the posts have said to pre-measure it into your bottles, but, you're not supposed to put the formula in the bottle before the water. So, unless your water is also pre-measured, I wouldn't suggest this.
I also take bottled water, but I don't keep it chilled. That way I don't have to heat it. Room temp water is fine.
If your diaper bag is on the small side, and you can't fit too many bottles in there (I send 4 to daycare with my daughter everyday), just have some extra water on hand in the car to rinse one out if you need it again). I do this all the time with DS's sippy cups.
Hope this is all helpful!
NOTE: when I send my daughter to daycare, I don't put the premeasured water in the bottles, b/c she can use her own. But I put a bottle of water in her bag in case we have to go somewhere after work. On the weekends though when we are out, I premeasure the water so it's easier and faster to make her bottle.