Engineering students at the Technical University of Dresden built a pedal-powered submarine out of carbon fiber reinforced cement back in 2002.
[Translated by Google from German]
"But the Saxon researchers wanted even more: "Our submarine, Yellow October 'is the first of its kind and has been through many successful dives after his trial at the nearby quarry," says the engineer. It is made up of seven flexibly interconnected units from carbon fiber reinforced fine concrete and can accommodate two persons. The thickness of the walls is slightly higher than the canoe with ten millimeters. Three layers of textile reinforcement, alternating with concrete layers, ensure compressive strength under water. Locomotion is achieved by means of leg strength on the pedals and power transmission to a propeller."
https://www.welt.de/wissenschaft/article1604762/Ingenieure-bauen-ein-U-Boot-aus-Beton.html
[Translated by Google from German]
"But the Saxon researchers wanted even more: "Our submarine, Yellow October 'is the first of its kind and has been through many successful dives after his trial at the nearby quarry," says the engineer. It is made up of seven flexibly interconnected units from carbon fiber reinforced fine concrete and can accommodate two persons. The thickness of the walls is slightly higher than the canoe with ten millimeters. Three layers of textile reinforcement, alternating with concrete layers, ensure compressive strength under water. Locomotion is achieved by means of leg strength on the pedals and power transmission to a propeller."
https://www.welt.de/wissenschaft/article1604762/Ingenieure-bauen-ein-U-Boot-aus-Beton.html