We have been to Dawn Beach and Great Bay Beach. They are quite different.
Our first trip (October 2002) we rented a van. We booked in advance through Hertz, about $60 for the day. There are many agencies right at the end of the dock. Check the major web sites. Get off the ship, walk down the pier, then veer to the right. It was easy and quick to get the car. We had maps down-loaded from the web, plus what they provided. We drove straight to Dawn Beach, no problem. There is a gate across the parking which was not yet open, but by the time we turned around and came back, it was. You park in a paved lot and walk down to the beach. There are steps and a paved path. There is an open air beach restaurant run by a European couple with nice grilled fish, drinks and sandwiches. There are restrooms and space to change. There is some fresh water for showers, but conserve, fresh water is VERY expensive on St Martin. There are vendors who will rent lounge chairs and umbrellas. The snorkeling here is only O.K., at least if you are experienced. The beach is wide, sandy and lovely, looking out to sea and toward Oyster Pond. There is a drop off and it is somewhat rocky, you'll be happier in the water with fins or beach shoes. The waves can be quite strong. Marigot is the major shopping didtrict on the French side. If you are looking for particular French goods, then shop here. There are still bargains to be had on French perfume (20% ??) and wine and the like, despite the poor dollar. The whole island is well worth seeing, at least once. You will not be sorry if you rent a car. The roads are narrow and a bit steep in places, but fine. We easily toured the whole island after spending the morning at Dawn and several hours shopping in Phillipsburg, you can park the car at a pay lot several blocks inland from Front Street.
Orient is much more built up than Dawn Beach. (It has been 10 years since we were there, so I won't compare.)
Great Bay is very convienient. Buy a water shuttle pass to the left of the end of the dock as you walk down the pier. Adults are $5, kids free. If you want to beach sit and shop this is the place. Others have expressed reservations about using a beach that is also a port but we found it to be very clean. No waves to speak of. (January 2004) Turn right or left when you exit the water taxi pier. My DH and the kids rented beach chairs to the right side of the beach, where there is much new, pristeen, construction. The rest of us had to simply walk back one block to all the shopping. If you want bargains and tourist junk, this is the place, not Marigot. Front Street of Phillipsburg is filled with jewelery shops. (Look for the lovely pink-amber and white and multi-color local topaz if you want local souvineers, rather than diamonds and tanzanite from Africa and emeralds from Colombia, cheaper, too.) There are lots of t-shirt shops and lots of local vendors selling interesting "tourist junk". There are also a few liquour stores with good prices. Try the local rum. There is a Belgian chocolate shop to die for. There are nice restaurants on the beach, we ate at a new one, bright lime-green, name that started with a "Q", lost the menu and forgot the name. Good, cheap pizza and wine. Nice folks. The bright yellow beach chairs and umbrellas we rented were "operating" out of here. You can use their restrooms. But it looked as if any of the chair vendors along Great Bay beach have some sort of "restaurant-restroom" connection. Pick what looks best to you or whoever gives you a "good deal".
If you like to explore, get a car and drive around. If you want to veg or shop, nothing is easier than Great Bay.
Carla