Welcome to the boards minniedew-
My favorite beach on St. Maarten is Dawn Beach. It's about a 10-15 minute cab ride from the pier. There is a restaurant (really kind of a food shack) there with restrooms, and you can rent chairs, floats, etc. right on the beach. The only thing is that the surf can sometimes be a little rough. It's on the Atlantic side of the island, so if there's a storm out in the Atlantic, it can be a bit choppy. Most of the times I've been there, however, it's been pretty calm and good for snorkeling.
Another good choice is Little Bay beach. This is just on the other side of Great Bay from the cruise ship pier, about a 5-10 minute ride. It's a smaller beach, but on the south (Caribbean) side of the island, and usually pretty calm. There are generally more locals on the beach than cruise ship passengers. There's also a Divi resort near there.
The most convenient is Great Bay beach, which you can see from the ship. You can take the water taxi from the pier to the beach. It's got everything you want, but it tends to be more crowded, as it is the closest to the pier. Also, there's a lot of traffic on in the harbor, so the water tends to be a bit more cloudy, which would really only matter if you were snorkeling.