Women's attire for an evening beach cocktail wedding

My boyfriend's sister is getting married in Florida in May, wedding time is 6 PM. Attire guide is "beach-y cocktail". I almost never go to weddings or dressy events in general, so I don't have much in my closet that might be appropriate. I'm open to buying a new item or two but ideally I'd like it to be something I could use in other situations if that's the case. So...\ I've got three ideas so far:\ 1. I have this yellow palm print bustier I usually wear over white wide leg trousers. Could do other pants/a skirt. 2. I found this white and navy striped linen dress that's my style. I could see myself wearing it to other summer-y events in the future, which is a plus. Not sure it's formal enough, although it is designer so I'd expect it not to look sloppy in person (maybe with some tailoring). 3. When I was a kid, I had a white-base floral cotton dress that I used for all spring/summer things. I've been thinking I should get an adult version to keep around, although I'm concerned a white base makes it off-limits for any weddings. Maybe like a cream or pastel floral?\ Do any of these options sound appropriate? Any NOT appropriate or better than others? I've got the spring to go shopping and welcome recommendations.