A strikingly modern spa stirs things up in Cardiff, Wales

"Where are they going to get the people to fill this place?" asks my cabdriver. He's just pulled up in front of St. David's Hotel & Spa, in Cardiff, Wales, and is gesturing at the massive modern structure. It reminds me of a ship being launched into the bay; obviously, my cabbie is not a fan of futuristic architecture. I suspect he also has never had a facial, and clearly he's not familiar with the real estate developers' mantra: "Location, location, location."

St. David's is part of an urban regeneration plan that is transforming Cardiff's gritty docklands into the focal point of the burgeoning Welsh capital. During the next two years, the 19th-century port will make way for Wales's own national assembly; a 72,000-seat sports stadium (to host the World Rugby Cup in October); and the Millennium Centre, the new home of the celebrated Welsh National Opera. All this promises to pack the trains leaving London's Paddington Station for the 2 1/2-hour trip to Cardiff.

Until then, the 21st century has advance billing at this 136-room hotel and its spa, opened in January by the new RF Hotels group. Hotel magnate Rocco Forte commissioned his sister, Olga Polizzi—whose own Hotel Tresanton made a big splash when it opened last year in Cornwall—to decorate St. David's. Poised on a promontory between the harbor and Cardiff Bay, St. David's has a spectacular prospect of the Penarth peninsula, as well as steel cranes, container ships, and small fishing boats squatting in the tidal mudflats. Rather than detracting from the view, the industrial landscape serves to complement the glass-and-steel hotel, with its soaring atrium and roof.

The lobby is a white temple of minimalism. Curved balconies rise above the reception area, where an oversize wire ottoman by designer Tom Dixon balances fellow Welsh artist Sophie Eynon's abstract paintings. Polished steel handrails and doors of white ash and oak add striped accents. Off the main lobby, the Tides Bar & Restaurant serves as a de facto pub for guests, who linger into the night after dining on monkfish in Penclawdd-cockle and chive broth or terrine of Brecon game and forest mushrooms.

I ride the elevator to my putty-and-cream room (No. 411), and step out onto the terrace to watch the tide fill Cardiff Bay. A brise-soleil made of white nylon sailcloth blocks the brisk North Atlantic wind. A wonderful Victorian brick tower, once the harbormaster's office, stands sentinel over the dockside construction; a Norwegian-style church bookends the same quay. Far out, a lone wind surfer tacks back and forth in front of the nearly completed dam that will shortly turn the bay into a 500-acre freshwater lake. I love the pungent smells of sea and marsh that waft up to my balcony, but I'm sure the water-skiing population of Cardiff won't miss them.

Donning a robe, I head for the special elevator that connects guest rooms with the spa. (There's nothing worse than having to skulk semi-clothed through a fashion-forward lobby where the staff is dressed in sleek Italian uniforms.) St. David's Spa, taking its cue from the seaside location, focuses on hydrotherapy, more familiar to Bretons than Britons. Like Brittany's thalassothérapie—spa baths containing minerals and seaweed extracted from the region's chilly coastal waters—the hydrotherapy pools at St. David's are enriched with marine nutrients. Guests gravitate to the navy-blue and marine-green pool room, and I don't blame them: a freshwater infinity pool there overlooks the bay. It's hard to decide whether to start with a gentle pummeling under the swan-neck massage fountains or an aquatic nap in the submerged whirlpool recliners. I opt for a quick detox in the cedar sauna/steam room, whose hypnotic lighting system changes color every three seconds.

The 14 treatment rooms are on the spa's second level, where guests sit in a relaxation room to await their appointments. I sink into a teak deck chair until Fiona, my therapist, arrives. A lap robe covers my feet, and I look out over the sun-dappled marsh, where gulls are playing aerial tag.

Unlike certain Continental spas, which cling to the outdated belief that hooking clients up to electrical gizmos—À la Bride of Frankenstein—will jolt away cellulite or facial wrinkles, St. David's steers its course closer to the New World. Here there's an American-style cardio gym and a full menu of holistic, hands-on facials, massages, and body wraps. Signature treatments include a relaxing underwater massage in a marine hydrotherapy bath; seaweed and mud body wraps; and a high-pressure jet blitz that feels like being sprayed with a garden hose, only exponentially stronger.

Fiona tells me that St. David's favors E'SPA services and products, a new therapeutic line from England that distills organic and wild plants in its essential oils and skin-care collection. After quizzing me about my stress level (high), skin condition (dry), and allergies (none), she waves several bottles of oil under my nose to determine which I find most appealing. The theory behind this simple olfactory test is that the nose knows best what the body needs-each potion is designed to induce different beneficial reactions. I like the restorative lavender and wheat-germ oil, which she uses to unknot my muscles.

Fiona also recommends the detoxifying algae wrap. After scrubbing me down with a spearmint-and-apricot-kernel body polish, she applies a warm coating of green goo and wraps me, sushi-style, in plastic and a thermal blanket. While the algae is being absorbed, she works a mineral-rich red-clay-and-watercress scalp mud into my hair, and massages my temples. She then applies pads soaked in cucumber extract to my eyes and exfoliates my face with a creamy marine phytoplankton polish. She finishes with a chamomile cleansing milk and tangerine-rose water spritz. At the end, she has to help me rise from the table to rinse off in an adjacent shower.

Back in my room, I slip under the bed's huge comforter for a little more "restorative therapy." I'm ready to sign on if St. David's ever slips loose of its mooring and sails off into the Atlantic. Maybe I'll invite that cabbie along. He could probably use a sea change, too.

St. David's Hotel & Spa, Havannah St., Cardiff; 44-1222/454-045, fax 44-1222/487-056; doubles from $288. Treatments cost extra, and one- to five-night spa packages are available.