Hey Surf,
The MT do not work on commission alone. They are employed by the contracted spa services company, not Celebrity (who I am referring to, sorry!). They receive a small "minimum rate", which is not nearly as high as North America's "minimum wage". I asked our last MT if she had to pool the gratuity, and she did not. She was looking forward to going home to see her husband and kids.
I don't know why males are not employed as MT. Actually, I was wondering if the MT on board are actually certified. I never asked, but assumed they were. Would be nice to have a male MT...the one we had on our last cruise had the tiniest hands. I'm a big guy at 6'7" and really wanted someone to get in there and work my muscles, covering more territory than she did.
I would tip 15 to 18%. A $20 tip for a $119 massage sounds about right. Be warned...they WILL try to upsell you on their product, lol.