I can’t speak to each individual cruise line. But, I think they all do it about the same. I would ask before I would cough up my money voluntarily as a “service fee” and hope that it gets to those that deserve it!
If you notice in the attached quote from a cruise critic article concerning Holland America they deduct credit card fees as well as paying some “bonuses” before dispersing funds fleetwide.
Holland America Line further explains that, "All of the Crew Appreciation and Service Charge payments made by all guests on all ships in our fleet are pooled, net of credit card transaction fees. The pooled funds are distributed throughout the year in the form of compensation, including bonuses, to crewmembers fleetwide who interact directly with guests and/or behind the scenes throughout every cruise, including those in the Bar, Dining, Entertainment, Housekeeping, Guest Services, Galley and Onboard Revenue areas."