We went to Coki Beach to learn PADI (scuba) diving - it was fabulous. Great beach for the others in your party to hang out on, and the dive center was well equipped and affordable. The cab ride was around $9 per person from the cruise port each way. You could rent snorkel gear for $10. 2 beach lounges w/umbrella was $20 for the day. They sell soda/snacks/lunch and gift shop items on the beach. The water is perfect, no rocks - calm (it's an inlet) and gorgeous. IT was pretty crowded but fun and casual. Loved the COKI Beach dive shop personnel - super friendly and accommodating. Coral World is adjacent but we didn't go there. Magen's Bay- awesome! Really cheap cab ride ($7 or so) to a national park with a protected inlet again for swimming. You can rent a chair and even get a slice of pizza. Great vibe, gorgeous calm waters to swim in - we even saw sea turtles there.
We have stopped here at least 4 times and this time finally did a city/beach all day tour. It was so worth it. We learned all about the history, but also went to several beaches and over to the French side to shop in the market. Everyone found something at the market to buy - some cool art and some typical things - we went to Le Galion beach which was windy but once you got in the water was fine. It would be ideal for kids. the Water is super shallow for quite a long stretch and they have a little platform where the kids can jump into the water. Umbrella with 2 chairs was $20 - but it was indeed really windy and we didn't hang out there all day. The water was so warm and very salty - we seemed to float like we were on an innertube - very different. Then we headed to Maho beach to see the airplanes land - this was like spring break frat party - super crowded with people drinking and trying to get blown over by the planes. the water was gorgeous though - but definitely a crazy scene.