It depends on a couple of considerations. How formal is it and what time of day? You can get a hint of the formality through the invitation and you can ask a family member.
The time of day is important also. An evening wedding is typically more formal.
This could be resort casual, with the type of clothing you would wear to a country club or out golfing. However, it could be cocktail attire, which is the little black dress or its equivalent for women and a dark suit for men.
So, this will take a bit of detective work on your end to fill in the gaps. If it were me and I couldn't find out any information, I would wear 'dressy casual,' which is the attire I would wear to the nicest restaurant.
Yacht Wedding & Reception
