I'd wear leggings, jeans, and short sleeves top but I'd also bring a light jacket if it's cloudy &@ the sea here in Oregon, it's always colder than the city. I have no idea what part of the world you live, but 68 degrees is comfortable for indoor temperature. If it's windy, going to the beach I wouldn't be wearing a skirt or dress, unless they're rayon &longer! Wear what ever you're comfortable with.
A sundress with a jean vest or jacket. A pair of gladiator sandals, in brown.
wear some decent and smart dress.
definitely a dress.