Spas at sea no longer underwhelm. Today's crop, some larger than 20,000 square feet, compete with the best found on land

Celebrity Solstice
Line: Celebrity Cruises
Passengers: 2,850
Itineraries: Caribbean, Mediterranean, Transatlantic
The Celebrity Solstice, scheduled to debut in December 2008, dedicates an entire category of cabins to spa devotees: Passengers in the Aquaclass staterooms—all a short stroll from the AquaSpa—receive two complimentary spa treatments; unlimited use of the spa's multisensory Persian Garden, hammam, and solarium; and exclusive access to Blu, a modern azure-hued dining room that emphasizes fresh ingredients and lighter preparations. The spa cabins also have mood lighting, aromatherapy kits, and a five-headed shower to prolong one's relaxation. Ships of this size can feel a bit overwhelming, making an intimate spa experience even more appealing. The Solstice, the first in a new class of cruisers for Celebrity, has a minimalist aesthetic that wouldn't look out of place in a boutique hotel, a top-deck lawn with bocce ball and an alfresco coffee shop, and glass-blowing demonstrations from the Corning Museum of Glass (800-647-2251; celebritycruises.com; seven-day Eastern Caribbean trip from $3,000).

Costa Serena
Line: Costa Cruises
Passengers: 2,930
Itineraries: Canary Islands, Mediterranean
This Italian-made ship has one of the largest spas at sea—the 23,000-square-foot Asian-inspired Samsara Spa. Not surprisingly, many shipboard spas focus on seawater-based therapies; here, the emphasis is on Ayurvedic treatments and Indonesian massages. No more will you have your post-massage bliss ruined by boisterous crowds on their way to the pool; as on the Solstice, the Serena has designated spa cabins nearby, 99 of them. The fare for these cabins includes a private spa consultation, a tea ceremony, and two treatments. Spread across two levels, the Samsara's sprawling facilities encompass a Turkish bath, a watsu pool, a thalassotherapy area, a rock sauna, a tanning center, a beauty salon, a teahouse, and a gym. You can avoid the buffet temptation altogether by sticking to the spa-only Samsara Restaurant, which serves healthy fare that won't throw your chi off-balance. A passing knowledge of romance languages will help on both ship and shore, since many of Costa's passengers are European—as are all of the Serena's routes. If your cruisemate is looking for a bit more stimulation, there's a Formula One race-car-driving simulator, plus four swimming pools, 13 bars, five restaurants, and an enormous outdoor movie screen (877-882-6781; costacruises.com; eight-day Croatia, Greece, and Turkey trip from $749).