Like many couples, we sometimes differ about the best way to spend a vacation. One of us -- him, usually -- would like nothing better than to hole up at a spa. The other -- often enough, her -- loves to be pampered, but doesn't necessarily want to be stuck in one location. Fortunately, we've found a solution that satisfies us both: spas at sea.

At least 70 per cent of the 158 vessels marketed in North America offer spa services, estimates Sarah Walker, a spokeswoman for Steiner Transocean Ltd., spa operator aboard more than 104 cruise ships.

Serious spa-goers, however, will want to book passage aboard a cruise ship featuring a purpose-built spa (a dedicated spa space incorporating water treatments and spacious relaxation areas to lounge in.)

"It's not just about going in for the massage and leaving -- it's the kind of facilities which you can spend the entire day relaxing at, if you choose to," Walker adds.

Norwegian Cruise Lines' SS Norway started this trend when it unveiled its 560-square-metre Roman Spa in 1990: a decade later, that figure has soared to about 50 full-fledged shipboard spas.

Seafaring spas are beginning to rival land-based, resort destination spas in both facilities and treatments -- as we personally discovered during a recent Baltic/Mediterranean cruise aboard the 758-passenger Seabourn Sun. Expanded and redesigned as part of ultra-luxury line Seabourn's $16-million (all amounts in U.S. dollars) Royal Viking Sun makeover last year, the 450-square-metre, Spa du Soleil exemplifies the increasing sophistication of floating spas.

Somewhere between Istanbul, Turkey, and Odessa, Ukraine, Paul checked in at the Roman-flavoured spa's reception desk, in preparation for a two-hour treatment named after Aphrodite, the Greek goddess of beauty. Once inside the Seaweed Suite (one of this spa's nine customized treatment rooms), a spa therapist named Amy goes to work on Paul.

It was Paul's first experience on a dry float table, a pallet that literally sinks into a bed of water, cradling the supine spa patron without getting him or her wet. (Imagine lying on a parachute or beanbag furniture cushioned with water, and you get the idea.) After 20 minutes, Paul rinsed off in the Seaweed Suite's convenient private shower before being rewrapped in foil and surrendering to a scalp-and-foot massage as the treatment continued its "detoxifying" effects. (Although the Aphrodite Treatment literature mentioned a minifacial, this ship's version included scalp and foot massage instead.)

A half hour or so later, we met up in Spa du Soleil's exotic-looking, ceramic tile-lined Rasul Chamber. Rasuls were allegedly used in ancient times to prepare women for the harems of the Middle East. Spa du Soleil's version of a rasul involves the use of three different-coloured "detoxifying" Austrian muds which we fingerpainted onto each other in a room which fills with steam emanating from a ceramic, samovar-shaped stove.

The Rasul Chamber's star-studded heavens eventually change to "rain", which -- theoretically at least -- washes the mud away. (Two hand-directed showerheads help the process immeasurably).

The ShipShape Spa aboard Royal Caribbean International's 3,100-passenger Voyager of the Seas also offers rasuls. Celebrity and Disney ships feature similar mud-based treatments without the steam or exotic sky cover.

Somewhere between Athens and Sorrento, Italy, Ellen also started a series of Spa du Soleil's cellulite reduction sessions (called ionithermie). During an ionithermie session, electrode pads are applied to targeted areas of the body in conjunction with certain physical applications of ivy, kelp and algae in order to emulsify the offending cellulite, which is then disposed of by the body's lymphatic system.

By the time our 16-day Seabourn Sun cruise disembarked in Genoa, Italy, Paul's face still glowed from an aromatherapy facial and we had each indulged in a second two-hour treatment that starts with a 15-minute soak in a herb-infused, sarcophagus-shaped tub in the Cleopatra Suite.

Jacuzzi-style jets massage pressure points on your feet, legs, back and neck while you lounge in a tub set into the private treatment room's elevated ceramic-tile platform. After being swathed in fluffy towels, you then climb face down onto a well-padded massage table for a potpourri of reflexology and shiatsu, followed by a scalp massage and 55-minute aromatherapy pressure point facial. We were both in wet-noodle heaven.

We found the Seabourn Sun's menu of spa services unusually impressive for a cruise ship, but sought an expert's perspective, too. "It offers more for the serious spa-goer than any other ship I know of," confirmed Bernard Burt, founder of the International Spa Association and co-author of 100 Best Spas of the World (Globe Pequot Press).

Though exemplary, it is not alone.

The 1,792-passenger Queen Elizabeth 2's QE2 Spa is known for its emphasis on thalassotherapy. Favoured in Europe, (some nations' health insurance actually cover it when doctor prescribed) thalassotherapy uses seawater to combat stress and manage medical conditions like arthritis and rheumatism.

The 976-square-metre QE2 Spa options include a body-temperate thalassotherapy pool and a Jet Blitz/Hydrajet treatment in which a trained technician turns a high power hose on your body to increase circulation and promote lymphatic drainage.

Radisson Seven Seas Cruises' 320-guest Paul Gauguin, also offers a jet shower at its Carita Spa (which also features three- and six-day spa packages).

Ellen tried this thalassotherapy treatment in France, and said it made her feel like a civil rights demonstrator in the U.S. South during the Sixties. If we've learned anything while going to spas together over the years, however, it's that favourites defy logic: the treatment one of us loved is the treatment the other hated.

Meanwhile, Celebrity Cruises' 1,950-passenger Millennium, launched in July 2000, currently boasts the largest thalassotherapy pool at sea. (QE2 Spa aside, thalassotherapy pools are also available on Voyager of the Seas and on Renaissance ships.) At 10 square metres, Millenium's seawater pool also includes ankle-to-neck massage jets and bubbling air "beds" which give bathers the sensation of floating on air. Other notable facilities include the communal-style "Persian Garden", a thermal suite composed of a series of scented gentle saunas, steams, and showers. (Disney ships' similar experience is called "the Rainforest"). Millennium's 2,325-square-metre AquaSpa is especially alluring for those who find Swedish massage a snore. Examples include its hot stone therapy which uses heated volcanic basalt stones and freezing cold marble stones as massage implements. Millennium's AquaSpa also offers services similar to the Cleopatra and Aphrodite treatments aboard the Seabourn Sun.

By contrast, Carnival's Nautica Spas offers a more traditional menu of massages and facials. One exception is its Noveau Yu Health Environment Capsul, a reputedly relaxing combo of aromatherapy, coloured lights and music delivered to a guest inside a temperature-controlled, oval-shaped chamber.

Services aside, the character and flavour of shipboard spas often differ, as well. Plantation Spa treatment rooms aboard Princess Cruises' Grand Princess feature ocean views. Crystal Cruises' spas were recently redone according to Far Eastern principles of design harmony called feng shui, complete with zen garden and wind chimes.

Small ships don't necessarily mean small spas, especially among high-end ships. At more than 900 square metres, Windstar Cruises' 312-guest Wind Surf's roomy spa emphasizes a holistic approach, including lessons in yoga, Eastern meditation, and couples massage. The Judith Jackson Sea Spa aboard Radisson's 490-passenger Navigator offers body massages performed by two therapists simultaneously (Ellen's idea of bliss.)

In response to the growing popularity of spa services among its passenger base, Silversea's two new 388-guest ships, debuting this fall and early next summer, will be 60-per-cent larger than the spas aboard its twin, 296-passenger ships. (Silversea Cruises' newly revamped spa program emphasizes Balinese treatments like a "Javanese Honey Steam Wrap" which incorporates cinnamon, ginger, coffee, sea salt, and honey.)

Those who primarily regard spas as a vehicle for getting in shape will love Carnival's state-of-the-art equipped NauticaSpas: some of the biggest gyms at sea. A rock climbing wall, full-sized basketball court and an ice-skating rink supplement Royal Caribbean's Voyager of the Seas' 1,395-squre-metre workout and aerobics facilities (which differ from Voyager's relaxation-oriented Solarium and ShipShape Spa).

Royal Caribbean, Carnival and Celebrity Cruises offer the most active fitness class schedules, says Steiner consultant Elaine Fenard. Most cruise ships also feature some form of local spa cuisine at every meal.

Spa service prices vary from ship to ship, even for the same treatment. We found the fees comparable to those charged by upscale spa resorts and tony day spas. Gratuities generally cost extra; some ships automatically add 10 per cent or more to your spa bill. (By contrast, the Seabourn Sun's "gratuities not expected" policy also applied to the spa.)

Range of services may be a more accurate barometer of a spa's quality than size: cruise ships sometimes count the beauty salon and gym in calculating a spa's overall area.