My experience is through working for Steiner Leisure.
Working in the salon you will receive commissions of about 7.5% on treatments and products you sell, and no wages.You will be expected to make at least half of your commissions through selling products and will be expected to sell products to every single guest. If you have a quiet day in the spa you will be expected to clean, sort out the stock room, do seminars, man the reception desk... but you will not be paid for any of this. The average hours vary depending on the opening hours. Usually you start at about 7am and finish at about 10.30 pm with one hour for lunch and half an hour for dinner. However within this 'free time' you will have to prepare your room for the next guest as clients are booked back to back.
Tips are not automatically added to the clients bill so what you get depends on the guest.
At the end of each cruise you will have to do a port clean, which means that along with the other members of staff you will have to clean the entire spa area thoroughly. This can often mean not finishing work until the early hours of the morning.