Underwater cycle.

Why not try and use a more efficiënt way of getting the power of your legs into the water.
The propellor is well developed, the cycling motion alows the upper legs to be used fully, this combination is promissing.

Where do we stand now?

My aim was twofold,

The mechanics are more complicated but below drawings show the progress I have made in the design. Ultimately changeable pitch propellors could be used, but at first contra-rotating propellors where the pitch can be changed out of the water would work quite well.

Contra-rotating propellors

One of the main problems with a large diameter propellors is that te resistance in the water will lead to rotation in of the underwater cyclist. As a helicopter, adding the tail rotor seems overly complicated and will increase the overall resistance.
Contra-rotating propellors can also increase the overall hydrodynamic efficiency of the propellors as the wash has less to no rotation. The mechanics of the pedal system can be seen in below drawing.

Pitch control

Changeable pitch propellors have been around for quite a while and to efficiently get the power of the legs into the water the pitch will have to be changed while "accellerating". Below drawing shows a mechanisem changing the pitch while the props are rotating

The image below shows a solution for changes the pitch of the propellors when out of the water. Remember that the pitch of the "lower" prop is quite a bit larger as that of the upper propellor.