Also, I think the window should take priority for another reason. It's the difference between a "definite" and a "maybe".

Once the water gets partway up the window, the window will definitely not work. The lateral pressure that the water puts on the window will render it immobile. Also, after some time underwater, the electronic controls will stop working (although this takes quite a while from practical testing [MythBusters!]).

However, the seat belt being a problem is a maybe. There are numerous accidents every day in which people simply unfasten their seat belts. It is quite probable that you won't even need to cut your seat belt in order to escape.

When in an emergency situation in the wilderness, you prioritize your needs by asking yourself, "Okay, what's going to kill me first?" The main obstacle to getting out of an underwater car is getting a window or a door open, not the off chance that your seat belt will be jammed. Prioritize accordingly.

Additionally, if you are thinking that opening/breaking the window first will let in a rush of water which will block off your air supply, consider these points:

• If the water is several inches up the window when you try to open it, you've likely waited too long to try opening it anyway. As stated above, the lateral force of the water pushing on the window will render it immobile. Your only choice is to break the window at this point.
• If the water is several inches up the window (not over your head) and you break the window, water will start rushing in. The water rushing in will not be over your head and impeding your breathing unless the force of the water starts throwing you around in the car and disorienting you.
• If the water level outside the window is over your head but you still have a large air pocket inside the car, then the chances of being thrown around and disoriented by the rushing current of water after you break the window is even greater. Leaving you seat belt fastened will prevent being thrown around. Additionally, as long as you are still poised next to the door (seat belt), you may have the option of simply opening the car door before you swim out. (Breaking the window while completely underwater equalizes the pressure very quickly, so the car door should open unless damage to the car frame and body have jammed it shut.) This will allow you swim out of the car quite a bit easier than trying to fit through the window.