The beach in St. Martin that you are refering to is Maho Beach. It is located just feet from the airport in St. Martin. Therefore, the airplanes fly over the beach very low when they take off and land at the airport. It is very impressive when you see a giant 747 landing there. I found a video of such a 747 landing at that airport on YouTube. I had a link to the video in another post here in this forum. But I will place the same link here:
Click Here To See Video of 747 Landing At Maho Beach
When your cruiseship (the Mariner) arrives in St. Martin, it will very likely dock in Phillipsburg (on the Dutch Side). I think that Maho Beach and the St. Martin airport are on the Frech side (I am not sure). But regardless, you can easily get a taxi in Phillipsburg that will take you to Maho Beach.
By the way, I heard great things about the Mariner of the Seas. It is one of the Voyager Class of ships in the Royal Caribbean fleet. Those Voyager ships were the largest cruise ships in the world until Cunard launched the QM2 and R.C. launched the Freedom of the Seas.