The Xiphactinus that approached Nigel when he was riding the Archelon got eaten by one of the Tylosaurus.
If you are keen enough, the small explanation of Nigel's adventure in Sea Monsters after the intro of the first episode shows brief footage of the prehistoric seas he's going to visit in the show in order from least to most dangerous. All the scenes shown belong to the actual show, except for the one of the most dangerous sea, that instead shows a Xiphactinus being quickly snatched by a Tylosaurus. This scene never appears in the Cretaceous sea segment, which means is possible it was initially meant to be on it but was cut for the final product. With that, is likely that the Xiphactinus who stalked Nigel when he left the Archelon's back was eaten offscreen by a member of the Tylosaurus family that appears in the climax of the documentary. A commentary Nigel makes about the presence of blood in the water after coming to the Ancient Mariner following the mosasaur attack also suggests this was the original intention, and there is some promo from the airing of the documentary in Discovery Channel that shows what it looks like deleted footage of the scene showing the fish eaten by the marine reptiles. The narrator is a time deity or a similar supernatural being that took interest in Nigel.
She came into contact with him and is the one who allowed Nigel and his crew to travel to the distant past. She didn't reveal herself in Chased by Dinosaurs, but decided to partially do so in Sea Monsters as the narrator of the documentary, closely observing Nigel's journey in her own realm while informing the viewers in the right moment.