Rather than fly from St. Martin to St. Eustatius (or Saba or St. Kitts), consider the ferry service . . . or maybe not. While they have a sleek 72-foot catamaran (MV Makana) that holds 150 passengers, it was in for repairs, so we were on a beater, a cramped storage locker of a ship. Pretty unpleasant. We raced from our St. Martin 3:15 plane arrival to get to Makana for the 5:15 departure (foolish plan); upon arrival at 4:25, the arrogant Makana worker seemed to enjoy telling me, "You're too late to board," even though the passengers were milling around. I had to grovel to get officially approved and then waited 30 minutes to board. They handed us barf bags, though the seas remained calm. With a stop in Saba, it was about a 3.5-hour trip.
At the end of our Statia visit, our boarding time for Saba, where we were to stay for a week, was 6 a.m. We arrived at 5 a.m., the required hour, but their office didn't open until 5:20. And the boat was late. I passed the time watching mountain goats and their kids miraculously scampering on the skinny front face ledges of the steep cliff across the street.
The ferry is $85 round-trip and adds to any travel adventure. But only book it if the MV Makana is making the run. And don't cut it close with your back-home flight departure. https://makanaferryservice.com/