The bareboat charter company will give you maps, instructions on what to watch for, and information on where you need to stay while you're out bareboating. But really, why would they worry about where you are?
The answer: because there are some surprises dangers if you venture outside the prescribed waters of the British Virgin Islands. Storms, pirates, and reefs all pose hazards that can do more than just ruin your vacation.
Perhaps the most common danger is overestimating your own skills with boating. Even if you have a boating license accepted by the charter company, do you really know what you're doing? Can you handle sails with the crew you have by yourself, or do they need more seasoning? If there is any doubt in your mind, a day or two with a bareboat captain to train everyone might be a wise decision.
Storms are something you may worry about, especially during hurricane season. For the most part, the British Virgin Islands are free of sudden squalls, although you can expect afternoon showers to be common during summer. If you charter your boat in advance during hurricane season, make sure you also purchase vacation insurance. It is rare that a hurricane hits this area without warning, but it does happen occasionally. If you're worried about the sky or wind, fire up the radio or head to shore. The charter company should be available to tell you what to do. Never try to ride the storm out yourself; while it's true that boats near shore can be broken up by the wave action, ideally you should not be in the boat at all during a hurricane. If you are, stay in constant contact with the charter company so they can send you help.
Reefs are another hazard, but if you stay within the boundaries of the map your charter company, you should not find them to be a problem. If you do find yourself out to sea during a storm, they may become a problem, and you'll want to steer well clear and downwind / downcurrent of them. Otherwise, just follow your maps.
Pirates are part of the history of the British Virgin Islands, and pirates still exist today throughout the world. They're just thieves in boats, after all. If you're in a boat by yourself on the sea, you're in a completely vulnerable position. The good news is that the British Virgin Islands are well-monitored by the US Coast Guard and the BVI's coast guard service, and pirates rarely venture into areas that are so heavily traveled. As long as you stay within the area prescribed by your charter company, you should be safe. Venture outside, and you may find some unpleasurable surprises.
Most piracy around the Caribbean takes place close to main shores: Venezuela, Guatemala, Columbia, Brazil (where at least one wealthy yacht owner was killed), and other poverty-prone areas are most likely to have pirates operating offshore. On the islands, you're more likely to find them near Trinidad. It is, however, a bad idea to pick up strangers without radio shore first. Pirates sometimes masquerade as castaways or migrant workers. In addition, picking up a stranger may get you in trouble with the British Virgin Islands, as they do have a problem with illegal immigration and therefore have very strict laws. To protect yourself, always check first before picking up strangers, and pull in only at docks that your charter company recommends.
Avoid trouble in general. You're on your vacation; do not let it be ruined by mishaps.