We keep a LifeStraw Personal Water Filter in each vehicle. These are also good for: Hiking, Camping, Travel, and Emergency Preparedness. If you buy multiple, pay attention to the pricing – sometimes the packs are cheaper, sometimes they are cheaper sold one at a time.
The filter assumes you have water to filter, so just having this isn't necessarily going to help. We keep water bottles in the vehicles (with a small amount removed in case they freeze).