[Christ]

I would much prefer something dark colored, natural, such as wood balls. Unfortunately, the amount of waste required to make those balls (it's not easy to make a sphere) efficiently would probably negate the savings from them for the average user, who doesn't keep the tub hot all the time.

For a public spa, I can see it making a difference, though.

I suppose the balls would be something approximately 1/2" in diameter, and another set approx 3/4" in diameter so that they would pack into each other. For vanity, they could be like bath beads which are infused with oils that, when heated, release a desirable odor.