We did Magen's Bay on our ownduring the last St. Thomas day, very easy, though we did not end up getting the return cab ride we expected--he was there, but waiting around for his taxi-truck to fill, so we jumped off and moved to open seats on a nearly filled taxi-truck farther ahead in the line. It was stunningly beautiful there, no waves, clear water like rarely seen. This time it looks like we are going to go to St. John and Trunk Bay on our own next time.
On Sint Maarten, we've never gone to another beach other than taking the water taxi to Great Bay beach, right there in Philipsburg, during two previous cruise ship stops on the island. Went to Dawn Beach during a week long stay on Sint Maarten a few years before that (sheesh, just realized,4 1/2 years ago!). Waves too strong for swimming there, we found. Great Bay is not the prettiest, to be sure, but it sure is easy, and after a while you stop being bothered by having to tell the locals "no thank you" every 30 seconds when they want to braid your hair or sell you something (they really are harmless and non-threatening). Have been thinking its time already to go to Orient Beach finally on our next visit.