> how easy is it to get from one side of the island to the other?
The island is pretty easy to navigate... there are only a few areas that you really need to learn.
Waterfront: This is the road that goes along the south side of the island from the airport to the cruise ship docks (havensight). Towards the center of the island, along this road, is known as 'downtown' which is where most of the tourist shopping is located. Blue Island Divers is located in Crown Bay Marina, which is fairly close to the airport off of waterfront.
Havensight: This is the area that most all of the cruise ships dock. Again, another pocket of tourist shopping.
Downtown: This is the area the most of the tourists visit to go shopping. There are lots of little stores along waterfront, main street, and back street.
Tutu: If you are going to buy cheap alcohol, k-mart at the Tutu park mall is the place to go. Plaza Extra, a supermarket, has the best selection of foods, but is not the cheapest or cleanest. This area is kind of the center of the island. If you are going to the east end, probably the easiest way to get there is to find Tutu, and follow the road going east... it will take you all the way to the east end.
RedHook: This is the east end of the island... along with the American Yacht Harbor (AYH). This is where Chris Sawyer Diving Center is located. The fairy to St. John is a very short walk from here. Marina Market, a supermarket, is across the street from the AYH, is fairly cheap (for island food) and fairly clean (for island stores). The food center, another supermarket, is about half a mile west, and is probably the cleanest supermarket on island. (I avoid peublo supermarkets like the plague... nasty, dirty, etc...)
Peterborg: This area is on the north side of the island on the way to Magans Bay. If you want to golf, or visit Magan's, this is the general direction you will head. Golf is at Mahagony Run Golf Course, which is just east of the Peterborg area and Megan's Bay is on the south/west side of Peterborg.
A good map of the island is here:
http://www.usvi-on-line.com/st/stmap.shtml
Ignore the road numbers... nobody (and I mean NOBODY) on island knows what road numbers go where... directions are typically given with respect to the areas I listed above.
If you are at the airport and want to get to Red Hook, the best way to go follows:
1) leave the airport. at the light, turn right (east) and follow waterfront.
2) drive drive drive, you will pass nisky, frenchtown, downtown, the legislature, the government buildings (two big mustard colored buildings, side by side), and finally end up at a stop light in the Havensight area. (On the map, it is where 30 meets 313) There will be an empty lot to the north and a lot of construction along with a Wendys on the south.
3) turn left at the light, heading northish... follow this road (313) until you get to another stop light
4) You are now at the bottom of Rapune Hill, which is up and to the right (east). Follow this road until you get to tutu. On the map, this is listed as 313 or 38. As you start to get back into "town" slow back down. The stop light at tutu can be a surprise if you are speeding along.
5) You are going to come to a big intersection with a stop light at tutu. The mall will be on your left. You want to turn right (going eastish/south) at the light. This is marked as 32 on the map.
6) Stay on this road all the way... you will pass the industrial area where they mix up concrete, past a bridge that goes nowhere, past a bunch of marinas, past the army reserve, and finally end up in Red Hook.
As far as diving goes, I would spread it out between Chris Sawyer Diving Center on the east end and Blue Island Diving on the south side. Both are excellent operations that visit much different types of sites. The east end operators tend to go to the same places as the St. John operators while Blue Island tends to get more of the wrecks and southern dive sites.
I have been living on St. Thomas for nearly four years now... I'm by no means an expert, but I can certainly help answer basic questions that you may have.