Most people arrive at Trunk Bay having already seen it a hundred times in photos, and the photos are all taken on the same kind of day. The bay is usually gentle. It is not always gentle, and the difference is worth knowing before you send a seven-year-old in.
What the water does
Trunk faces north, which is the whole story. In summer the north shore is typically calm and clear, and the bay behaves like a swimming pool with a view. In winter, north swells periodically wrap in and turn the shore break into something that will knock a small child over and stir the sand up enough to make snorkeling pointless. It's not dangerous so much as disappointing — and it can change within a day.
If the sand at the waterline is churned and the water near shore looks milky, the visibility offshore will be poor too. That's your signal to swim, skip the mask, and come back another morning.
The snorkel trail, honestly
The underwater trail runs out toward the cay on the left side of the bay, marked with submerged plaques. It's a genuinely nice introduction to reef for someone who has never done it, and it's shallow enough that nervous swimmers can stay comfortable. It is also not the best snorkeling on St. John, and people who've snorkelled elsewhere sometimes come back a little underwhelmed. Treat it as a well-signposted first reef rather than the main event.
Current tends to run along the bay toward the far end rather than out to sea. Get in near the cay, drift, and walk back up the sand — swimming against it the whole way is how people end up tired and cross.
With kids
The middle of the bay is shallow, sandy, and the calmest water you'll find on that coast. Lifeguards are usually on duty. There are restrooms, showers, and food, which on St. John is not a given, and it's the main reason families make Trunk the anchor of the day.
Go early. The ferry-and-taxi crowd builds through the late morning, and the parking lot fills. Our day-trip guide has the ferry timing worked out, and the honest review covers whether the entrance fee is worth it at all.
If you'd rather have the calm water without the ferry, the east end of St. Thomas is where we live and where both our places are. Current open dates are on the deals page.