DeniseM is such a flower child. In addition to what she brings, we always like to bring one or two pair of underwear.
While we haven't quite pared down to just a carryon, we certainly pack lighter now than we used to. Technology has helped quite a bit. For instance, the laptop has been replaced with an iPad, and our Nikon D50 has been replaced by a reputable point-and-shoot (Canon PowerShot) that fits into a small purse. Using post-production editing software included in most cameras, we can get great quality prints, even posters, by cropping our point-and-shoot photos v using the zoom.
I also second the recommendation for snorkel gear (if that's your thang), a soft-sided cooler, and a rashguard shirt to keep you warmer in the cool water.
I also bring a variety of plastic baggies, from one or two large kitchen garbage bags to quart sized baggies. They weigh next to nothing and can be used from everything to hauling home wet bathing suits to packing sammies for lunch.
I also bring a variety of spices in well-marked baggies ("Steak seasoning" or "garlic powder") because we like to grill out a lot and it's cost prohibitive to buy an entire bottle of, say, Montreal Steak Seasoning or Old Bay for one meal.
We now use a backpack as one of our carryons, so that we can use it as an actual backpack when we go hiking and day trips. It's an essential item to us.
And finally, we always bring our Aquapacs, which are waterproof storage bags with lanyards. We prefer the small size, which is big enough to fit our cellphones, keys, wallet and credit cards. We never leave our valuables unattended on the beach or in the car; instead we put it in an Aquapac and bring it with us when we get in the water.
http://usstore.aquapac.net/explore-product-range/waterproof-phone-cases.html