It depends on what you want to do.
Personally, we want to have a leisurely day and do what we like to do when we like to do it. So, we prefer to rent a car and do our own thing. As you come off the pier, turn right and there are several car rental kiosks. Rentals are cheap, ranging from $35 - $85 depending on the vehicle you want and the time of year.
We like to drive to Marigot and do some shopping at the wonderful outdoor flea market. Then we'll make our way to Orient Beach and have lunch at the Bikini Beach Bar & Grill. Then we'll go over to the other side of the stone jetty where the nude beach is and enjoy a wonderful day at a beautiful beach skinny dipping. (Don't try to picture that - you'll hurt yourself!) But there's a bar on the beach and there will be anywhere from a couple of hundred people to several thousand people, depending on the time of year.
Of course, if you don't care for nude, then stay on the other side of the jetty and you can still enjoy a beautiful beach with lots of activities. Just know that you're on the French side and you will definitely see topless. (But hey, at my age, I enjoy topless more than I use to!)