When they purchased this beach getaway in Water Mill, NY, five years ago, the homeowners had two requests for designer John Bjornen: timeless rooms and kick-off-your-shoes comfort. With two young boys, a baby due soon, and extended families bringing the guest count to 20 over the weekends, it's no surprise the couple requested crowd-proof livability—and Bjornen delivered with classic, casual rooms in a palette of marine blues. Like the unfussy sunporch, above: The rustic storage-chest coffee table came pre-distressed, the beige rug camouflages any dragged-in sand, and the vintage reed furniture is almost indestructible. "It's stylish as well as kid-friendly," says Bjornen, who continued the theme by upholstering sofas in weather-resistant Sunbrella fabric and choosing accessories with more personality than pedigree. The result: a chilled-out vacation home as laid-back as…