Xiphactinus
Above all else, Xiphactinus was a great swimmer, able to speed towards or away from virtually anything else in the seas of the time. It may have been able to leap above the waves at times to help dislodge parasites from its skin. It was not however immune from attack. If injured, its large size meant it was easy to spot and could become prey for sharks, mosasaurs, and other Xiphactinuses.