Three things are cheap in St.Barths, and just these ones: alcohol, cigars / cigarettes and gas ;-)
You need a car. Don’t ask if you can walk or take a bicycle…You need a car and a 4W car! The rest is bullshit.
French and English are spoken everywhere.
Euro and $ and accepted everywhere. No problem with credit cards.
Voltage is 220V, 60Hz. French plug converters are required.
Cell phone: nothing special if you have an international subscription.
DO NOT forget to bring something against mosquitos. There are really annoying…and quite strong in St.Barths (although all the hotels are treated two times a week against mosquitos).
Time zone: as US East Coast, 5 to 6 hours difference with France
Typical tropical region for the daylight: all the year between 06:30 and 18:30, so that the people are waking up early and going to bed quite early in the evening.
Weather: also typical tropical weather. Some clouds in the morning, a little very pleasant wind, somtimes some suddend tropical downpours (short and normally duting the night), 28-30°C during the day, 22-24°C during the night, 25-27°C in the sea :-)