I believe a lightweight flowy dress as a beach cover-up is essential because it's the easiest and most put together outfit for the beach. I like to accessorize with a waist cinching belt to keep my figure in a shapeless dress, and a hat and sandals are a must.
Dress: Urban Outfitters. Belt: Anthropologie. Hat: World Market. Sandals: Gap.
(Swimsuit not shown: Victoria's Secret)