For most personal watercraft, buoyancy is a virtue. But the new $100,000 Cayago Magnum sinks by design. Propelled by an emission-free, battery-powered jet-stream engine, the Magnum can take divers to depths of 130 feet when equipped with an optional buoyancy-adjusting weight. Both underwater and on the waves, the sea scooter can travel at speeds up to 12.5 mph. The Magnum’s 30 lithium-ion batteries provide up to four hours’ worth of power to the engine, and an LCD includes readouts for a navigation system and a sonar location system. Cayago, of Stuttgart, Germany, will build only 100 of the scooters.