Wikipedia
The PowerSwim is a device somewhat like two pairs of long thin airplane wings, one pair at each end of an axis. The axis is fastened to a scuba diver's shins by straps round his legs. The longer pair of wings (about wide or a bit less) is at his hips and the shorter pair is at his ankles. The wings rotate limitedly on axles near their front edges, and thus on upstroke and downstroke they propel water backwards. It is claimed that the length of the front wing lets it operate outside the cone of wake that starts at the diver's shoulders. It is claimed to let a scuba diver or frogman swim much faster (250%) than with swimfins for the same amount of bodily effort, if used correctly, and being not motorized, it makes no motor noise to be heard by hostile hydrophones, but noise would occur if the front wings are allowed to hit the diver's hips at end of upstroke. It works somewhat like a penguin's or plesiosaur's side-mounted flippers. Its estimated cost is less than $500. The diver uses it by moving his legs up and down together, letting the knees bend and straighten.
It was developed by DARPA in 2007.