All ichthyosaurs were totally sea-going; it is known from some fossils that all ichthyosaurs gave birth to live young in the water.

Ophthalmosaurus icenicus  swam in a fish-like manner using its powerful tail for propulsion, at between 2.5 and 1.0 metres per second and its large broad fore-paddles for steering and manoeuvering. 

What did they eat?

Ophthalmosaurus had quite long jaws but had only a few small teeth so it probably did not feed on large prey that had to be impaled and gripped. The diets of some ichthyosaurs are known from preserved stomach contents which consist of the hooks from tentacles of squid-like animals called belemnites. It seems likely that it was a squid-eater, able to dive deeply to feed on them – just like sperm whales do today.