From bladefish.net: Take the Seajet underwater and the fun begins. With this personal aquatic propulsion device from Bladefish there is no need to fin - just hang on tight and zip through the water at up to 5km/h for an hour on just one charge. 

Think of the amount of air you normally expend fighting a current underwater. Now think how much more time you could spend sub-aqua if you just had to hang on to the Seajet. The top of the range, 5,000 model weighs less than 5kg; the base model, 1,000, is even lighter. Seajets also float, so there is no risk of losing them underwater.