Platypus Swordfish 40

On the surface, the Swordfish 40 (US$2.8 million) looks like a luxury tender (its styling was inspired by the 1970s Citroën SM) with a transparent cockpit. But in its semisubmersible mode, the central hull pod drops 2.1m below the water, effectively becoming a see-through submarine for six. While passengers enjoy the view below – and the underwater sound system – the boat cruises silently on electric power at six knots.