The batteries will have to be encased in waterproof material.
Probably quite expensive and not easy. Will also make the cars more expensive and heavier. Heavier means less range.
Tough to do, because the batteries get very hot as they are used. The battery pack has its own ventilation system that blows air into the pack and the metal box (the pack) is usually louvered to remove the heat. If the ventilation system stops, I would imagine the computer shuts the car down immediately, otherwise it would be negligence on the manufacturer’s part.