Magens Bay is on St. Thomas, not St. John, and it's a very nice beach. It's popular with the cruisers though so it can get a big crowded. Sapphire Beach is another nice beach on St. Thomas. There are more secluded, private beaches if you hunt them out. Trunk Bay in St. John is a nice beach, but again if you get away from the touristy areas, there are just as beautiful and less crowded beaches. And don't forget Hawaii! It has some of the bluest water I've seen! I've only been to Oahu, and again, if you get away from the tourist traps and go to the other side of the island you'll find white sand and crystal clear water, and very few people. Good luck planning your trip!