I checked
WindGuru for the BVI and the north swell will have 13-second interval; that gives it a
LOT of power. I think that a north swell of 5-6 feet will prevent snorkeling on the northern beaches of Anegada. Even if the reefs contain the waves, the water will be roiled up and visibility will be sub-optimal. I haven't been in the anchorage at Anegada during a north swell but if conditions get bad you won't be able to leave through the narrow channel. Fortunately, it is just a short hop to and from the North Sound - the only best refuge from a north swell in the BVI.