OneSpaWorld, a spinoff of Steiner Leisure, is the company that operates the spas on 19 different cruise lines. Among the cruise lines OneSpaWorld operates the spas on are Seabourn, SilverSea, Crystal, Regent, Oceana, and Azamara.
I do not know how OneSpaWorld applies the mandatory gratuity on the other cruise lines.
but my guess is that it is similar to the way it is done on Seabourn.