Many people go to St. John and rely solely on taxi service. This is certainly an option, especially if you are in walking distance to the bars and restaurants in Cruz Bay. That said - we have always rented a Jeep to get us around the island so we have flexibility to pack our beach chairs, cooler, etc.
St. John is VERY casual -- almost 2/3 of the island is National Park! Comfy shoes/sandals, swimsuits, coverups, etc. In town, visitors are expected to be fully covered. For men, think shorts, t-shirts, short sleeved button down shirts (Scott has recently discovered a love for Tommy Bahama island button downs and will likely be seen wearing one every day). For ladies... casual sundresses, shorts and t-shirts, etc. Keep it simple! Sunscreen, bug spray, cameras, snorkel gear. What NOT to pack: villas will provide towels, beach chairs, coolers, etc. If you are booking through a hotel, please check with them to learn what they will offer for you. Chairs are available for rent at certain beaches.
There is no airport on St. John. Plan to fly into St. Thomas (STT) and hop on a ferry to Cruz Bay. St. Thomas has two ferry ports: Crown Bay (~10 min from airport) or Red Hook (~30 min from airport).
Cocktail attire for the ceremony! Jackets and ties are not required (and honestly, not recommended). Men — khakis (or any slacks; just recommend lighter colors for the heat!) and button downs… short sleeved button downs are totally fine, shorts are honestly fine too if it feels like pants will be uncomfortable with the weather.