One clichÈ of traditional dinosaur wisdom (I mean human knowledge
about dinosaurs, not the philosophy
of dinosaurs

) was that theropods (e.g.,
T. rex) were incapable of swimming, while herbivores (especially the duck-billed and long-necked varieties) were very good at it. As DarkHououmon stated, this was not the case. There are a number of known fossil trackways of theropods kicking their way through shallow water, so evidently these animals swam sometimes. The bodies of most living land animals are neutrally buoyant (they neither float nor sink in water), meaning that swimming comes fairly easily to them. Seeing as theropod skeletons were highly pneumatized (filled with hollow spaces containing air), they would have floated like corks.
Back to the subject of swimming theropods in LBT, we don’t even know how the
Giganotosaurus got to the island in the first place. Obviously, it either used the land bridge (in which case it would have had to have arrived before the gang did) or swam across, but no indication is given of which one. Either way, it would have had to arrived fairly recently, as Chomper believed his parents to be “the only big sharpteeth around here,” and something as big as that
Giganotosaurus would not go unnoticed for long on such a small island.
Even if the
Giganotosaurus was overpowered by the current at the end of LBT V (though for all we know, it could have been swept all the way back to the mainland :unsure:), I can’t say we’ve seen any indication that LBT sharpteeth can’t swim. The raptors caught in the damburst in LBT III seemed to have been quite good at it, seeing as they made it to shore before they were even swept past the point where the leafeaters climbed out of the streambed. And in the TV series, Chomper seems to have no problem participating whenever the gang plays in water. And as far as the original Sharptooth is concerned, I think that he probably drowned not because he couldn’t swim (as Littlefoot had presumed), but because the rock the gang dropped on him knocked him unconscious, or at least breathless.