In modern deep-diving marine mammals, such as whales, the outer shell of the bones is porous  This has the effect of making the bones light enough so that it is easy for the animals to swim back to the surface after a deep dive  The outer shell of the bones was also porous in the ichthyosaur, an extinct prehistoric marine reptile
We can conclude from this that ichthyosaurs were deep divers
In most modern and prehistoric marine reptile species that are not deep divers, the outer shell of the bones is porous.