You can use roaming service (TravelPass for Verizon or International Day Pass for AT&T and T-Mobile) which automatically activates when abroad, or an international SIM card can be purchased more cheaply from local carriers (TIM, Vodafone, etc.).
Boats will be arranged from the mainland to the ceremony venue on the island and from the island to the reception.
Hotels in the area do not offer room blocks and in fact prefer staying away from them. You can book rooms through their respective websites. When making reservations, be sure to indicate the number of guests staying in the room because hotels actually use the size of the party to assign rooms. Breakfast is usually included, but conditioner is never provided. Please bring your own supply.