If you are staying long term and responsible for utilities be prepared for a horrifying electric bill, even if you don't run the AC much. Ditto for water (our combined bills ran close to $400 over the 6 months we were there).
Planning my return soon, here is what I learned in 6 months on island:
1. Greet everyone with good morning, afternoon, or night. Do not use 'good evening'. Do NOT just jump straight into 'I would like a ...' with someone in a shop/restaurant, it is considered very rude and your service will suffer. Actually one of the things I miss most from the island! The mainland seems very gruff in comparison.
2. Draft beer is very expensive, and only available at the brew pub on the boardwalk or 1844 tavern a block or two off the boardwalk. The ONLY place to get draft Guinness is 1844, and it is $9/pint. Worth it when you haven't had a real bear in 4 months though.
3. 4:00pm at Deck Bar for happy hour. Say hi to the crew from SCUBA for me. Do not let Simeon buy you drinks, he likes some nasty tasting stuff!
4. Miriam's on hospital road for tasty Mexican food. Galangal for good Thai. Chicken Shack if you have a car (mid island near UVI) and want some amazing fried chicken.
5. Dive the pier at night. A LOT. Do the shallow version for sure and the deep one if you have two nights. Do some shore dives at Carambola for a nice long day. Wall is a trivial sim out and the shop on site will run your tanks to and from the fill station on an ATV. Do the night dive off the north shore for some unique bioluminescence if your trip falls on a new moon.
6. If you rent a car and go to the beach, just leave it unlocked with the windows down with no valuables in it.
7. I found St Croix to be very safe, but then I am 2 meters tall so folks tend not to bother me when smaller game is plentiful. Do be aware and careful, it isn't any more dangerous than other places, but it IS very small so what come there is tends to be a lot more visible and feels closer to home than it would in a city of 50,000 on the mainland.
8. Drink some mamajuana... Yummy!
9. The people are fantastic, hang out at the bars and talk to locals. Be careful because you won't want to come home.
10. Don't kid yourself and think that you will just sign up for a Tec40 class and be done after that. Ended up going all the way thru TriMix with SCUBATec and had a blast doing it.