After the sea and see they will either drop you off in Philipsburg or at the ship. Great Bay beach is right there in Philipsburg. So you could get off there, do some shopping and go to that beach. If you return to the ship, you can take the water taxi back to town/beach for $5 adults, kids free. Cabs are available at the dock too which can take you to another beach as well.
Our tour stopped in Marigot first then the semi-sub but some tours go the other way. You could probably stay in Marigot and check out the french side beaches if that was your last stop then just catch a taxi back to the ship.
We just checked out the few shops at the pier after our tour then enjoyed a Heineken on our verandah while our DD napped.