THIS $16 MILLION PRIVATE ISLAND IN THE FLORIDA KEYS IS LITERALLY MOSTLY UNDERWATER Only about 0.7 of its 6.5 acres sits above the Atlantic. The other 5.8 acres are privately owned seabed, including the reef and snorkeling grounds surrounding the island. It sits a quarter-mile off the coast in the middle Florida Keys, close enough to reach the mainland in minutes but with no bridge. You arrive by boat at the 100-foot deep-water dock or land on the private helipad. On the land, there's a three-bedroom main house, a one-bedroom guesthouse, a pool, a fishing pier and about 5,000 square feet of living space. Solar, wind, batteries and an on-site freshwater system keep it running off-grid. The same family has owned it for more than 30 years and rented it for as much as $17,500 a week during the holidays.