I would say, invest in a good escape tool and training.

I have trained with the helicopter ditch simulater in a pool and trained in a car mounted on a cradle they could turn around. it gives you new perspective on what is needed.

I have a blanket and towel in the car, but I don't expect to get it out of the trunk if my car is in the water.

Some tips on escape tools:
- Must be able to grab it from your seat
- Must have a proper mount, that stays in place
- Big and brightly colored

For the window part:
- Simple escape hammers with no moving parts; a good one is reliable (super cheap ones might fall apart), but once underwater it's hard to get a good swing.
- Centrepunch based rescue tools have a spring that can fail, but doesn't require a swing. Do put it in the corner and do use it with force.

For the seatbelt with V-cutters, do the following for best results (in my experience anyways):
- Pull the belt tight
- Cut at a 45 degree angle.