Learn to snorkel. It's not hard, and it's the best way to explore the treasures that lie below the surface.
With snorkel, mask, and fins in hand you'll be able to slip into another dimension, one that us air-breathers don't spend a lot of time in. You can spend a lot, or a comparative little (in the $100-range), for this gear, or you can rent the gear for about a third that much for a week. How much you spend on a purchase, or whether you should rent, should be measured against how likely you are to get wet again after leaving the park.
If you already love to snorkel, or are bitten by the bug once you arrive at St. John, don't overlook a day-trip to Salt Pond Bay on the southwestern side of the island. This arguably is the best beach for snorkeling, lying about, and fleeing the crowds. Get there before 10 a.m. for a prime selection of shaded spots, some with picnic tables, to serve as the day’s base-camp. Swim either out to the rocky point below Ram Head or out to the nearly submerged rock outcrops near the bay’s mouth for great coral displays – Elkhorn and Staghorn corals, as well as brain corals, which look like, well, brains.
Pay attention to the seascape before you as you head to either place, as we've spied nurse sharks lurking on the ocean floor and sea turtles grazing on the underwater vegetation. Barracudas also watched us swim by.
Another great spot for snorkeling I'm told is Virgin Islands Coral Reef National Monument on the far east side of St. John. (This is one of the reasons a 1-week stay wasn't enough.) The 12,708-acre monument was designated in January 2001 and encompasses "submerged lands within the 3-mile belt off of the island of St. John. These waters support a diverse and complex system of coral reefs, and other ecosystems such as shoreline mangrove forests and seagrass beds that contribute to their health and survival." The only part of the monument accessible by land is in Hurricane Hole. To get to Hurricane Hole, follow route 10 from Curz Bay to Estate Hermitage.
If you have the time, and the budget, consider a "snorkeling tour" of the waters surrounding the islands. These can be arranged through private outfits that typically use sailboats to carry you to off-shore reefs. These reefs could be more colorful than those closer to shore, as the waters off-shore aren't quite as warm and so aren't "bleaching" the corals. Plus, a sailboat ride is a great addition to any vacation.