I carry a case of bottled water in the back of my personal and work vehicles all the time. Any water you carry will probably need to be rotated, poss. 2 to 4 times a year. An ice chest is a handy way to carry the case of water, or many other types of supplies in your car.

addition: for scuba diving in hot weather I would take along one or two 2 litre soda bottles which were washed out and refilled with water, then frozen. That gave you a good supply of iced water on a hot summer day. Even with repeated use, never had any problem with the bottles. If you want to prep these bottles and transfer from the gallon jug of drinking water from the grocery store, that may work.