It’s obscure but there was such a contraption trialled by the seals and cia in the 1950s http://www.hisutton.com/CIA_Water-Air_1958.html

Here’s a pedal powered smuggling submarine from the 1940s http://www.hisutton.com/Swiss-Pedal-Powered-Smuggling-Submar...

> French company Seabike

They have a .fr domain and a showroom in Cannes, France but the company is headquarted in Italy:

  PARITET SRL, Via Giovanni da Cermenate 3, 22063 Cantù (CO) Italy

Also, the French version of the website is riddled with enormous errors, like "For traveling light" translated as "Pour voyager lumière", which does not make any sense and isn't even grammatically correct (the proper translation would be "Pour voyager léger").

The whole thing does not inspire a lot of confidence. Is the product real?

> The nominal mode enables motion through the water at 3.6 km/h, and for speed-seekers, the SEABIKE can reach a maximum of 7.9 km/h – much faster than normal swimming speeds or even flipper-assisted swimming.

https://www.nauticexpo.com/prod/seabike/product-68606-564117...

Pretty fast, but "superhuman"? For short distances Michael Phelps can swim faster :)

Would this device be even better if it has the newer efficient propeller design called MX-1 Sharrow Propeller ?

https://www.boatus.com/expert-advice/expert-advice-archive/2...

At the 2020 Miami International Boat Show, Philadelphia-based Sharrow Marine introduced the culmination of a seven-year research and development project called the MX-1 Sharrow Propeller. Unlike every prop that's come before it, rather than blades, the MX-1 has loops of metal attached to the hub.

How does this change the dynamic? In a nutshell, much of a prop's inefficiency can be blamed on the blade tips, where vortices and cavitation (commonly called tip vortex cavitation, or TVC) form, creating turbulence and sapping efficiency. Simply put, the loops on a Sharrow have no tips. The net result is an efficiency gain of between 9% and 15%. But just as important, eliminating the cavitation vastly reduces vibrations and noise and makes for a smoother, quieter boat ride.

Company president Greg Sharrow tells us that the development of the MX-1 can be credited to music videos.

"I was trying to solve the problem of reducing unwanted noise from drones while filming live music productions," he says. "I've always thought it would be cool to use a drone to get cameras closer to subjects and film them from onstage, but you can't use drones for music broadcasts because they're too noisy. I knew that most of the noise comes from the blade tips and is caused, in part, by tip vortices. So, I'd have to find a way to eliminate them."

"Seabike says the prop turns slowly enough that you can safely use it at the local pool"

Felt a bit iffy about this claim. But looking at the research it seems cadence lowers normally when cycling under water.[0] Fun device. I wonder which pedals would be best for barefoot riding(?). Maybe those strapped ones fix riders like to use.

[0] https://ieeexplore.ieee.org/abstract/document/23706