That's exactly the tool I would recommend. If you can't get one, a window punch (a steel tool about the size of a pen) small crowbar (12 in.) or a regular hammer. A knife in your glove box or console will solve any possible seatbelt issues or removing a child from a car seat in a hurry. Another thing you can do before the car submerges is to get on your back and kick the upper corner of the side windows with your heels. (the side and rear windows are tempered glass and will shatter, unlike the front laminated glass)

If you impact the water, your battery will not immediately die. The car will not sink right away, but it will go to a nose down angle because of the weight distribution. It will generally float out to deeper water, so escape early. If you have power windows, roll them down asap. Do the same with manual windows. Don't forget to unfasten your seatbelt.

If the worst happens and the car falls from a bridge or is swept away by flood and sinks before you can escape, or even worse, it inverts, you must stay calm. Under water, the hammer might be your only hope of breaking the glass. The other option is to let the car fill with water until the pressure equalizes, and then open the door. If you can get the window down, swim out with the current.

Lastly, for parents. Your instinct will be to save your child first. With the car in a nose down attitude, you will be "climbing" uphill to the back seat. In that situation, leave the windows up. Have your hammer (it also has a recessed seatbelt cutter) and a knife. (unless you can unbuckle the child really fast) When you have the child, smash the back window, (always hit them in the corner) and GO, because the car will sink rapidly after losing the air bubble.