The Music Man, Professor Harold Hill, said it clearly: "You gotta know the territory."
Visitors to St. John can get confused easily, but rarely lost. After all, there are only three main roads. To help, there are a number of maps on the Internet that can be, if not useful, at least worth a look before you step foot on the ferry dock. Here are links to a few, and some comments.
Produced by anonymous people who claim to have "A combined history of over 30 years on St. John," this is primarily a promotional vehicle for rental villas in Coral Bay. While it does show streets and bays for the entire island, it's not very detailed or interesting. This kind of looks like a weekend experiment seeing what could be done with Yahoo Maps.
Virgin islands Map is also promoting businesses on the island, oddly only in Cruz Bay. There's no detailed map of Coral Bay. It's also out of date – Chilly Billy's is gone, Grapevine Salon's at the Marketplace now, not Mongoose. But the map does give you an overview of the kinds of the kinds of businesses there are downtown and roughly where.
This map is offered by Anthony Finta who runs Virgin-Islands-On-Line. It doesn't show streets, downtown businesses, or much of anything other than the names of the bays. Which is not a bad thing to have. You're not going to use this map to go shopping, but it gives you an idea of what the island looks like and where the beaches are.
National Park Service
The National Park Service's map is the biggest and most detailed we've found. It shows beaches, ruins, mountains, bays, and more. It's also quite detailed. It uses Flash to display and is a little difficult to figure out how to zoom and pull out. But best of all, the map can be printed. As such, it's a must to bring with you.