Hamptons guys tend to dress like schmucks -- unsurprising, since many are schmucks, or aspiring schmucks. So when you hit the beach, scrap the pink pants, and outfit yourself in Loden Dager.
LD's an NYC-based design collective whose casually outdoorsy collection will make you look like a Northeastern seaboard native, rather than a guy who throws summer collar-popping parties. Their shirts, sweaters, jackets, pants, etc, are all 60s Ivy League-y, tempered with earth tones that'll suggest you still smoke drugs. The material's top-notch (Japanese denim, ludicrously soft cotton), cut for a normal, somewhat athletic man -- neither too boxy, nor tailored for Strong Islanders crippled by a pathological need to flash triceps. Special consideration's given to durability (reinforcing inserts, etc), so your wardrobe should survive even the most boisterous clambakes.
Whatever you go with you'll know you're getting the best: LD eschews superfluous variety in favor of a perfect representative of each category. This ups the odds that a fellow weekender will show up dressed just like you, but at least you'll both look respectably copycat-ish, instead of like a pair of fuchsia cocks.