Xiphactinus had a dark blue back and light silver belly to camouflage it from above and below. Sharp teeth at one end and a powerful tail at the other combined to make it a formidable pursuit hunter.
Xiphactinus cruised in surface waters of the oceans. It caught other large fish (swallowing creatures up to 2m long whole) and was prepared to have a go at seabirds on the surface, like a floating Hesperornis.
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.