Anguilla is MUCH less rough than going to St. Barts, and the ferry ride, particularly from Marigot, versus by the airport, is much shorter. Unless the seas are particularly rough, shouldn't be an issue going to Anguilla. The good thing about the public ferry from Marigot is that you don't need reservations at all, so just watch the weather and if the seas are really rough, don't go. St. Barts, you can almost always assume the trip OVER will be rough, because of the prevailing currents. The trip back from St. Barts is normally much calmer.