The dress that makes the biggest splash at this summer's soiree might not be a dress at all. Swim cover-ups can make chic, comfortable and relatively inexpensive cocktail dresses, and they pack down to almost nothing in a suitcase.

The season's top styles — one-shoulder silhouettes, metallic hardware and rich jewel colors, among them — make the easy transition from day to night because they're infused with a little everyday glamour.

"Many of the fashion trends for spring are translating to swim: There's ethnic with tribal prints, patterns and animal prints, embellishments with more 'organic' trim and beadwork, ruffles and ruffle trims. Floral prints are very happening in spring and beachwear," says Colleen Sherin, fashion director at Saks Fifth Avenue. "I do think the designs in swim are becoming more sophisticated. Swimwear designers seem influenced by ready-to-wear, and a lot of ready-to-wear designers are designing beachwear. You can have Gucci to Pucci."

VERSATILE VALUE

Carmen Marc Valvo made the leap from eveningwear, his specialty, to swimwear, and says it was a seamless process. "Unlike sportswear, eveningwear is very exposed and swimwear is even more bare. They're similar. They both have a lot of draping, and I can work with tricot as if it were chiffon."

And the same tricks he employs to create flattering gowns work here too, Valvo says: The obi sash narrows the waist; halters flatter the neckline; ruching is shorthand for camouflage.

Saks' Sherin notes that cover-ups are making up at least half of the swimwear business for some brands. "It's an important part of the beachwear business. I do think that reason is the added value or the ability to wear two or three ways."

There's also more flexibility in sizing, notes Rosemarie DiLorenzo, co-founder and creative director of Swimwear Anywhere, the manufacturer of designer licenses such as Valvo, Marc Jacobs and DKNY. "You think about how many purposes it serves — and how many types of women it will fit."

The inspiration for so many of these cover-ups come straight from the catwalk, such as Michael Kors' chain-link belts and clear plastic panels, and Jacobs' ruffles.

But you're not paying runway prices for these pieces. For example, Valvo's dresses typically sell in the $500-$1,000 range; a dress from the swimwear collection is closer to $150.

"Fancy, fancy kaftans — which can be $600 with sequins — are the best cocktail dresses of all time, if I do say so myself," says Kiran Rai, creative director of the Satya Rai label.

"The fit is sexy when the draping is done in a way that's form-fitting," she says.

GOOD ALL DAY LONG

Rai even wears her scarf-print kaftans to work in the middle of Manhattan. "I started this collection because I traveled so much. I wanted to be able to rumple up my clothes and wear them to so many different places."

The cover-up that can also be a dress also is never fully sheer, nor shapeless or cinched just under the bustline. "You need to shop for it like a dress. You need to try it on and get the right size," Rai advises.

And don't forget the accessories, adds Sherin. Wooden or gold bangle bracelets, sunglasses, a sharp tote and thong sandals complete the package, she says. "It's really a complete head-to-toe look for these cover-ups. We're seeing our customer really likes creating a look around the swimsuit — and then wearing it all day long. You really don't need much else if you're going away to a resort or beach location, and you can wear this to lunch, cocktails and dinner."