I suppose that sharptooth was a Liopleurodon. There is no 100% certainty as that species looks similar to Kronosaurus. Mosasaurus and Tylosaurus had a distinctly shorter snout than the sharptooth in the movie and Geosaurus' snout was too pointed. Rhomaleosaurus might match the movie sharptooth's look as well, but I think this species neck is a bit too long. Teleosaurus snout again is far too thin and pointed (similar to the snout of modern day Ganges Dolphin).
As for all these statements about the look of the species it is important to base them on the look of fossils rather than artist interpretations of the species look. If one searches through the internet it is not unlikely that there are pictures of all these species vaguely matching the looks of the LBT 9 sharptooth, but not all of those pictures are really based on what the fossils tell us.
So Liopleurodon and Kronosaurus are the main remaining possibilities. My reason to guess it is the first one of those two are on the one hand that Liopleurodon was probably the larger of the two (the beast in the movie was huge!), though there is still a debate among paleontologists on how large both species really were. There were bones found that suggest Liopleurodon could have been as long as 25 meters, but they are not absolutely certain whether or not those bones are in fact from Liopleurodon (they're just fragments, no complete sceleton). Liopleurodon was also kind of popularized, shortly before LBT 9 was made, through the BBC documentation "Walking with Dinosaurs". I could well imagine that the makers of LBT decided to include a specimen of the kind that had come accross so impressive in that well-known documentary.
