No matter where your spa might be - cruise ship
The whole purpose of the spa is greater health through relaxation, through the relieving of stress. From the time the client walks through the door, there should be a sense of that peace, carried through scents, through soothing music, and through the appearance of the facility, including the look of the furniture. The aesthetic style of your spa furniture is almost as important as the feel of it, and not just to the clients. The attendants and practitioners who work there would certainly work better when enveloped in a pleasant environment.
Sometimes, spa furniture is the attraction. For instance, a dry water massage bed is furniture that also serves a function for the client with little intervention from an attendant. This could be a foam pad upon with the client lies, and is then covered by a plastic sheet. Water is sprayed from above to give a shower massage without getting the patient wet. Or you may go for the model made much like a waterbed - but instead of being simple filled with water, the massage bed is heated and supplies jets of water and air to massage the tension out of the body of the client.
A centerpiece of most spa experiences is the massage. And massages require massage tables, of course. Make sure yours is well-padded, easily cleaned, and has proper head support. After that, the sky's the limit. Your table might be simple and use pillows and pads for positioning the client, but more than likely you'll want an adjustable table that can be moved and manipulated however you need. It doesn't even have to be physically moved by the massage practitioner. Instead, you could opt for a hydraulic table which can be positioned with a few presses of a button or twists of a knob.
Special spa furniture also comes into play when considering manicures and pedicures. Starting simply, you might just need a good foot stool for pedicures and a little table for the arm for pedicures. If you want the best, however, you'll have chairs specifically meant for the care of the hands and feet. Your ideal chair may even provide a massage as your happy client has her nails attended to, or a footspa for soaking and cleansing the feet before or after a pedicure.
Lastly, when it comes to Spa Furniture, make sure you don't forget someone as important as your clients - the practitioners who work there. Many of them will spend a great deal of time sitting, especially if they are involved in facials, manicures, or pedicures. You'll want to model what your spa practices by providing them with comfortable chairs that are ergonomically sound. Not only will it keep the practitioners healthy, it will keep them happy, as well.