In Jurassic Park Evolution 2 it's a known issue that dinos get stuck sometimes, most notoriously the creatures in the lagoon, like your Mosasaurus (the huge dino-croc). While this sounds like a minor inconvenience since you're not directly interacting with the dinosaurs, it also means that your dino will not be able to do anything else - including feeding, which will lead to starvation.
If you have a med-unit assigned to their lagoon, they will keep your Mosasaur alive for quite a while, medicating it endlessly, but eventually the health will be so low that it's beyond medication and it will starve.

Apart from the heartbreak (if you're emotionally invested in the creatures under your care), this also has an effect on your rating (guests don't like suffering and dying dinosaurs), and a potentially major impact on your finances because the Mosasaurus is one of the more expensive creatures to synthesize and incubate and with your park losing it's star dino guest numbers will drop.

So here's the simple solution on how to unstuck your Mosasaur (and potentially other dinos):


You don't even have to exit the game, just load the save you just created.

I know this goes against every instinct we gamers have, because if there's a problem we want to stay there to fix it. But the solution is to let the game fix itself.

Before that, I tried everything else - diving in and trying to push the dino away (which of course couldn't work because you don't have a body in the game).
Manually operating the drone and trying to push the dino away with it (didn't work, either).
Trying to delete the shark feeder it seemed to have gotten stuck on (it wasn't possible to delete it, because the game told me the feeder was being used).
Manually assigning ranger- and med-teams in the hope that they would somehow magically fix the issue (which of course they didn't, but I was getting desperate).

I might have had a couple of other... creative... ideas, but none of them worked, either. So just save and load your game and that should reset your dino in it's environment.