A couple thoughts.

You're probably not going to wear flotation gear, a dry suit, or anything intended for in-the-water cold protection, while you're driving.

While my emergency kit includes a minimal capability to throw a line to someone in the water, you probably don't want to keep a throw bag within arm's reach in the car. I think the utility of a throw bag is very limited. For it to make sense, one would need:

  • To go in the water by accident
  • To be sufficiently uninjured to use a throw bag
  • To be close enough to shore to have a chance to throw it far enough
  • To have a sufficiently good throwing position to throw it a useful distance (meaning, at least one arm free, and preferably standing at least partially above the water)
  • For someone to be there who is able and willing to pull you to shore
  • And for all these things to happen when you can't swim to shore

I'm not saying it can't happen, but I do think it's better to spend my limited funds, weight and especially space on something else.

I keep a Leatherman Raptor, which cuts seatbelts and breaks car windows, in the center console where it can't get out of arm's reach for me in the driver's seat. That was an inexpensive tool to buy and almost unnoticeably small. I suspect it's more likely that I'll use it to help someone else than to help myself.