What can I wear to the beach? (please read explanation)?

I'm going to a beach party this weeked, it's all day. I'm going to wear my bikini, but I don't know what to wear over it/when it gets colder. I was thinking of a sundress, but I don't want people to see my cutting scars on my thigh. I still want to look good... So... Any advice on wha I should wear?