Built for late Microsoft co-founder Paul Allen in 2003 and recently refitted, the Octopus is one of the largest private vessels in the world. Measuring 414 feet bow to stern, it has eight decks, 13 cabins with room for up to 26 guests, and space for 60 crew. Onboard you'll find dual helipads and hangars, a recording studio used by Mick Jagger, a glass-bottomed observation deck, a dive center complete with a submarine, a basketball court, a cinema, a huge pool and jacuzzi, and a library. It's powered by a pair of hybrid motors that give it a top speed of 19 knots and a range of 12,500 miles when cruising at 12 knots. It costs $17 million a year to operate, but if you've got the money to buy it, that shouldn't pose much of a problem.