There are casinos on Dutch Sint Maarten but not on French Saint Martin. Fortunately the ship docks at Philipsburg on the Dutch side.
Most of the downtown Philipsburg casinos are rather seedy-looking with the exception of one... the Coliseum Casino, which is NOT on the main drag, but is probably a $3 taxi ride from the ship, looked fine from the outside. But you know what they say about judging a book by its cover...
The nicest casino on the island is reported to be the Princess Casino at Port de Plaisance.
www.princessportdeplaisance.com