Ophthalmosaurus
Calculations suggest that a typical Ophthalmosaurus could stay submerged for approximately 20 minutes or more. The swimming speed of Ophthalmosaurus has been estimated at 2.5 metres per second or greater. Even assuming a conservative speed of 1 m/s, an Ophthalmosaurus would be able to dive to 600 meters and return to the surface within 20 minutes.