Riding a bike on dry land is a great cardio, abs and legs exercise. Add water and it becomes extra challenging for your core, shoulders, and legs because you have to work them harder to prevent floating away. Try underwater cycling. Aqua spinning classes have been around in Europe for years, first introduced as a rehab exercise. Aqua cycles have no wheels, gears, or knobs. They are attached to the pool floor and use waist-high water for resistance. As you can imagine, the faster you pedal, the harder it gets. You have to get very strong core engagement as the back and the core stabilize.