Everyone should have something like this in their car.
In the event you find yourself and your car sinking in water, you NEED a device that can slash through seat belts / shoulder harnesses and shatter a window so you can escape. It is not uncommon for car doors to be impossible to open when under water. The only method of escape is generally to break a window.
This inexpensive, lightweight and convenient product from resume is a perfect solution. Available from Amazon.com for only $17.95, this could save your life in an emergency.