We spent a day at Shoal Bay East a few years ago, just to take a break from St Martin. It was a wonderful experience. Probably the widest, longest, and most beautiful white sand beach I have ever seen.
We did rent a jeep for the day. It was about the same cost as round trip cab fare. It was fun to drive around the island and check it out.
It seems like the ferry cost was about $100 round trip for both, the car was $50, and we probably spent another $150 for chairs, lunch and drinks, so no, it's not an inexpensive day but we still talk about the pleasant quiet day we had.

Bring your passport!!