That tennis balls are the worst balls for your dog, especially if they like to pick it up off the beach or garden soil or something similar. The combination of wet and sand/soil attachment, turns the ball into sandpaper in effect.
Our friend found this out when she had to take her dog to a dog dentist. She was told about the wearing away of her dogs teeth and when asked, confirmed that her dog does use a tennis ball, pointing out that that is the type of ball that comes with a chukker.
He recommends a floating rubber ball, designed to fit the device. Having looked around, here is something similar to what he is referring to here.