Hertz can book you a rental car for pickup at the port by contacting hertzres@sintmaarten.
Hertz Head Office | #82 Airport Road, Simpson Bay, Sint Maarten, Phone: 1+721 545 4541 | Fax: 1+721 545 4540 or Phone: 590 590 77 77 77 Fax: 590 690 66 66 66. I got a rate of $90/day ($60 for the car and $30 for tax and concession fee.

There's also Coastal at https://www.coastal.sx/ they're quoting $74.75 (all in) for a compact (Hyundai Accent or similar) 866-978-8361 or email info@coastal.sx