Earlier we wrote about Gerber's GDC Hook Knife, a keychain-mounted device intended to cut you free from a seatbelt. We also mentioned that that's a statistically unlikely need for your average motorist to have; if your car is submerged, the larger problem is going to be getting out of the vehicle before it fills with water. Back in the days of roll-down windows, you could crank them down if you got to it quick enough—once the water pressure got too great, it would trap the windows firmly within their rubber seals and you'd be screwed. However, with today's power windows, you've got the issue of the motors shorting out if you've driven into the drink.
Every year some 400 Americans drown in their cars. (That's just 0.01% of the population, but man, what a terrible way to go.) If you can't get the window open in a sinking car, you'll need to break it. And yes, there are objects designed specifically for that task, like the carbon-steel-tipped LifeHammer Plus and LifeHammer Evolution.
There's a bunch of panicky media-frenzy-style videos about them on YouTube, but we managed to find a calmer video that actually lets you get a good look at the products' design and functionality:
Unsurprisingly, LifeHammers are manufactured in Holland, a country crisscrossed with canals. If you Dutch text and drive the way we Americans do, I bet there's one of you driving into the water every fifteen minutes.
Speaking of phones, cars and water, wasting your time calling emergency services while your car is sinking is a good way to guarantee your demise. You may want to read Popular Mechanics' "How to Escape a Submerged Car."