Personal underwater propulsion devices are becoming increasingly popular as technology becomes smaller and less expensive.  A uniquely simple and effective example comes from Chinese startup Sublue Tech which has just introduced their WhiteShark MIX Underwater Scooter ($350).
A lightweight, hand-held propulsion device that uses a balanced two propeller system, the WhiteShark is easy to operate; the two triggers must be depressed together to start it and releasing one stops the props.  To keep your fingers safe, the propellers are caged, and if the scooter ever escapes your grip, it will slowly float back to the surface.  The whole thing weighs only 6.5 pounds and it has only one speed, so in calm water, it will pull the swimmer along at 3.45 mph. The Sublue WhiteShark MIX is rated to a maximum depth of 130 feet and the battery provides an hour of propulsion per charge.  It even has a GoPro mount on the bottom for documenting your undersea adventure.  Details in the video. 

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