Put cake mix, pudding mix, sour cream, eggs, oil, vanilla and water in mixing bowl. Mix at low speed for 30 seconds until combined, and then raise speed to medium and beat for two minutes, scraping the side of the bowl as needed.
Spray the inside of a 4-6 quart slow cooker with cooking spray and pour the batter into the slow cooker.
Cover the cooker and cook on low setting for 6-7 hours until the cake is a little puffy. (NOTE: Some of the newer model slow cookers seem to cook this cake faster than the older models, so if you have a newish slow cooker check your cake after 4-5 hours or so to see if it is done.)
Scoop the warm cake with a spoon into serving dishes and top with ice cream (I like serving it in mason jars.)