Portable water bike - GIF

[lest21]

I guess if all you are wanting to do is combine a little cardio with your swimming then this might be ok. the motion to peddle the propeller creates a lot of disruption in the water requiring you to expend an excessive amount of energy to propel yourself through the water. If you are wanting to actually go somewhere while conserving energy, then get yourself a pair of US Divers Jet or Rocket fins and learn to use your ankles with a slight scissoring action with your legs.

Those were the fins to have back in the day, still have a pair in my old dive bag. Probably just rubber mush by now.