Why did it have such large eyes?

Ophthalmosaurus had the largest known eye relative to its body size in the animal kingdom. It has a ring of overlapping bones called a sclerotic ring in the eye socket. These bones protected and supported the eyeball and size of the ring indicates that the eye was very large. It has been calculated that Ophthalmosaurus’ eye had a low f-number of 0.8 to 1.1 so that it could see in very dim light. This is one line of evidence that suggests it was able to dive deeply. Calculations involving body mass and oxygen usage indicate that Ophthalmosaurus could reach easily 600 meters deep and come back to the surface in 20 minutes.