That's why I ALWAYS...I mean ALWAYS...carry a Victorinox "Fireman" knife.
Jas a speical blade for cutting seatbelts, nylon etcetera.
prisoner6
The problem with those, and every other Victorinox-type design is that the blade can and will collapse back into the handle with only a little upward pressure on the blade. This usually means a cut hand at best, and a partially amputated finger at worst. And then your hand is slick with blood when you may need a good grip to get out of the car....
The other problem is that a Swiss Army Knife type always requires two hands to open, something you may not have in a crash, as well as manual dexterity (which goes out the window with your adrenaline rush/stress).
I prefer the one-hand-operation and lock-back design of the Gerber EZ Out knives, which are about $20-30 and are actually made in the USA! I carry the EZ Out Skeleton: http://www.gerbergear.com/product.php?model=6751
The serrated edge *will* cut through a seatbelt in just a couple of strokes. Been there, done that. One is in each of my vehicles and clipped in my front right pocket.
If you prefer one with a blunt tip, specifically for cutting seatbelts, try this: http://www.gerbergear.com/product.php?model=6971
If your car was sinking in water, isn’t there a possibility that you might mistakenly use that implement to open a bottle of wine instead of cutting your seatbelt?