Semi-formal is a step down from formal wear or cocktail attire. It is more relaxed. Men do not need to wear a full suit, and women do not need to wear a formal dress.
There will be a spot where you have the option to leave your shoes and go barefoot, or wear the provided flip flops for the ceremony. If you chose to keep your shoes on, we recommend ones you do not mind getting sand in,
You will know you were given a plus-one if the invitation was addressed to you and the plus one's name. Unfortunately, if you did not receive a plus-one there is no wiggle room.