This is not your average yoga. The main difference is that you're out on the water doing your poses, and have to work extra hard not to fall in.
Paddle board yoga classes are offered on Lady Bird Lake by Say Om Yoga, and it's definitely not the same experience that you'll get inside a yoga studio.
Those brave enough to try it will go out on the water on a specialized paddle board on their knees at first. Then the yoga instructor will get them going with their first move. What makes it so difficult is that they are not just trying to keep their balance for the yoga pose, they are also trying to keep themselves from falling in the water. The fear of falling in is why the instructors make people just jump in, so that you can focus on the workout.
For more information you can head to their website at http://www.sayomsouthaustin.com/.