The Underwater Bike Race is an annual charity fundraiser held in North Carolina.
Researchers from the University of Tsukuba have conducted multiple studies about the possible benefits of cycling underwater as opposed to doing so on land.
One of their studies focused on the cardiovascular response to cycling underwater, the study found that pedaling cadence and resistance both have significant impact on the cardiovascular response during underwater cycling.
[1] Another of their many studies on the subject focused on how the intensity of these exercises impacted respiratory function.
They found that breathing during underwater cycling is actually shallower, which could result in chronic effects on respiratory function if underwater cycling is executed long term [3]