Poseidon Rex (2013)

When a college couple on vacation in Belize hire a boat for a snorkeling tour, they find a body floating in the middle of the ocean.  Bringing the man onboard, they discover he is still alive.  He tells them his name is Jax, but isn't keen on providing any other information.  They head to port for medical attention.

After some minimal bandaging from a Marine Biologist, Jax tells them he is a treasure hunter and needs their help.  The lure of treasure ensnares half of our college couple, while the other half glares and stays on the dock. Meanwhile on the other side of town, some silly looking gangsters wonder where Jax is and if he's stolen the treasure.

What no one realizes is that the explosion that injured Jax and killed those he was diving with, has released an ancient amphibious dinosaur, and it's hungry.  From that ridiculous plot and the silly cover, you can tell this isn't going to be much of a film.

This would be right at home on the Syfy channel.  The monster is inconsistent in it's size.  Sometimes it appears to be walking, yet the ocean is supposed to be far deeper than it's little feet could reach.  Watch for the underwater scenes where people are talking to each other even though they each have a breathing apparatus in their mouths.  Impossible!