Some think perhaps crocodile, which would be more likely. Though, as far as mesoszoic marine reptiles go, a mosasaur is the most likely to have persisted, simply because it went on until the very end of the cretaceous. Plesiosaurs would be responsible for zero sea serpents IMHO because they are completely un-able to raise their neck to look like a long-necked sea monster, even a hpothetical seal would be more likely to raise its neck that far, or, unfortnately, perhaps simply an atmospherical illusion, created when an animal surfaces at a distance.