It could be theoretically possible to clone a marine reptile, within the blood-sucked-DNA-geologically-preserved world of Jurassic Park. It'd require the animal to come close to the shore, and ideally into a mangrove swamp type of area (where parasitic insects are particularly common). That may have happened - many species of sharks enter estuaries and swamps to give birth in a shallow, food-rich, creche-like environment. Dolphins prefer to give birth in shallow water too, and need to stay near the surface for extended periods when they do.
Given that marine reptiles all needed to surface to breathe, there would have been opportunities for parasites. The question is whether they could break through tough, scaly skin (which in the world of Jurassic Park they can) and whether they could tolerate brief submersion or detach at water level.
Trouble is, they all reproduced by live birth, not by eggs. It'd make them very difficult to recreate, even if you had the DNA - what has a uterus equipped to handle a huge swimming reptile?
