If, while you're visiting the BVI, you do decide to book a day sail (and I highly recommend you do) here's what you can expect. Depending on your destination for the day, your boat will be departing either from Village Cay marina in Road Town, or Soper's Hole marina at West End, usually around 9:30am. Your captain and crew will give you a quick safety briefing, and off you will sail.
If you're going to Virgin Gorda and the Baths, get comfy, because you've got a two-three hour sail before you reach the Baths and Devil's Bay. If you're going pretty much anywhere else (Cooper Island, Norman Island, Jost van Dyke) you can expect about an hour's sail before the boat moors at your first snorkel stop. When you stop, your crew will let you know the best areas to spot wildlife, and any dangers you may encounter in the water.
After 45 minutes' or so snorkel time, you will head to another mooring where either your crew will prepare lunch, or you will have an opportunity to go ashore for lunch, depending on the boat. The afternoon will usually see another snorkel stop before heading back to Tortola for a return time of 4:30 or 5:00.
There are maybe six or seven boats on Tortola offering day sails. Here are a few boats I've worked with before:
Mystique, by Voyage Charters. A beautiful new catamaran, Mystique's prices do not include lunch, but guests are offered the opportunity to go ashore to Soggy Dollar (on Jost), or Pirate's Bight (on Norman) for lunch.
Aristocat / Lionheart, by Aristocat Charters. Captain Steve will regale you with tales of his years in the BVI, and will happily share information on BVI history and geography.
Kuralu Kuralu is known for the delicious gourmet lunch served on board, along with Captain Gary's willingness to go wherever his guests request.
White Squall II is the only monohull day sail boat in the BVI, for a true sailing experience.