Tom,
I'll preface my reply to you with my normal disclaimer that I'm the owner/operator of St. Croix Ultimate Bluewater Adventures (SCUBA), a dive shop on St. Croix. Just want to make sure you know of my vested interest in answering your questions.......
Do all the shops in St Croix use big boats?
Yes and No. There are shops with big boats and some with smaller boats. Although we use a big boat (our main boat is a 42' Delta custom dive boat) that is certificated for 24, we limit it to 14 to make sure that our dive staff can give each diver the personalized attention for which we've become known (check out Trip Advisor). A "big" boat doesn't necessarily mean a "cattle car" type of operation you may have experienced on other islands/destinations. 4 of the dive operators here use larger boats but most of them limit the number of divers they take.
From what I can tell from their websites it appears they are all set up for giant strides out the back of the boat. I’m a big guy, have very bad knees and don’t like to walk to the back of the boat geared up, I would much rather back roll from where I don my gear.
That's not a problem. We have a couple of options for you.
1. Put your gear on in the water. You get in, and our Captain will hand the gear to you. Our dive staff will help you into it if you need.
2. Though we do normally use a "giant stride entry from the stern swim platform, we also have two side exits available for back roll entries.
3. Our "adventure entry". You get a good running start toward the stern and we'll trip you as you get to the swim platform. We like to see at least a one-and-a-half flip on your way into the water on this one.
Is it best to book all dives in advance or wait and do it there?
It never hurts to book in advance. A week or so is good. Again, we limit the number of divers on our boat(s) and want to make sure that we have space for you. We have two different boats that serve different areas of the island. You can easily switch between boats to take advantage of the diversity offered with diving the north shore (walls and reef) and west shore (wrecks, reef, and Pier).
-What are the “must do dives”?
Salt River Canyon, both east and west walls are my favorites and they're completely different. East wall has more marine life while west wall has an awesome topography that's not to be missed. The farther west you go along the north shore the steeper the walls are. There are some phenomenal reef dives along Long Reef (the 2nd longest barrier reef in the Caribbean). The Butler Bay wrecks are very nice. There are 5 shipwrecks within about 100 yards of each other. It's not like they're in hazardous water that precipitated their demise, rather they were all sunk intentionally to be dive sites. Typically you would either dive the two deep wrecks, or three shallow wrecks, depending on the choice of dive site for the day. You do not want to dive on St. Croix and miss the Pier. It is THAT great of a dive. We dive it as a shore dive at night and either shore or boat dive (dropping divers at one end and picking them up at the other) by day. DO NOT MISS DIVING THE PIER!
-What are the non dive attractions on the Island?
There are a lot of things for you to do when you're not diving, or for non-divers who might be traveling with you.
The island is 28 miles long and 8 miles wide. The east end (also the eastern most point of the U.S.) is dry and arid, while the west end has a rain forest. In the rain forest you can go hiking, horseback riding, and do ATV tours (I think this was mentioned in an earlier reply to you), and be sure to stop by the "jungle bar" and see the Beer Drinking Pigs.
There's a botanical garden which is a great place to check out. The Whim Museum is a restored Danish sugar plantation that does tours. Cruzan Rum and (now) Captain Morgans rum both have visitor centers. The one at Captain Morgan just opened and though I haven't been there yet I'm hearing excellent feedback from those who have been there.
There's plenty of great snorkeling around the island, lots of wonderful beaches, and the normal non-diving watersports you would expect (jet skies, parasail, etc).
And by all means check out the wonderful assortment of restaurants that we are blessed with here. We have everything from 5 Wine Spectator Award winning restaurants to McDonald's, and everything in between.
-Any and all other tips welcome/wanted.
Rent a car and (if you have one) bring your GPS. Several of the major car rental companies are at the airport but there are a couple of local companies that do a great job and have better rates than the big guys. Most will pick you up at your hotel (or here at the dive shop). Check out Judi of Croix, Olympic, and Centerline. They're all online.
Get ready to be wowed by St. Croix. This is one of the coolest, friendliest, and most laid back islands in the Caribbean.
A previous reply said to bring your passport and that's good advice. It's not needed ( because we're a US territory ) but it will streamline clearing customs and immigration on your way back home. By the way, that all happens on St. Croix not in some mega-customs facility back on the mainland. It's quick and easy here. Don't forget that you can bring up to 6 bottles of liquor back with you as long as one is a local rum. And, the airlines won't charge you for checking the liquor box as baggage.
If I can be of any further assistance in giving you any info about St. Croix don't hesitate to let me know. I've lived here over 25 years so I'm starting to get the feel for the place.