Wear a short or long dress on formal night, I think that the fact that you worry about it means you'll be more that appropriate. Just stick with formal colors, don't do summer day-time colors. I have one long and one knee length and then my casual formal are just my "cool style".
As for your "day trips" just wear something that you can wear over your bathing suit, maybe take a extra pair of panties for the dry effect. Hope this helps.
I have a pair of really thin white linen draw-string pants and a white long sleeve shirt in case I've gotten too much sun.