A program I was watching last night involved a search for the lusca, a crryptid of the Bahamas, said to dwell in its mysterious Blue Holes and to be powerful enough to drag a human under the waves. The creature is said to have the face of a shark and tentacles of the kind associated with octopuses.
After much analysis, the TV investigators didn't find an actual lusca, but by studying its characteristics, came to the conclusion that it was an octopus of some kind. When swimming forwards, with its tentacles streaming behind, the face of the octopus can have a definitely sharklike look about it.
The investigators indicated that the blue holes themselves are connected to the sea by labyrinthine tunnels. This ruled out a shark, which would not be able to negotiate these.
The Blue Holes are literally that. They are on the Bahama Islands and are caves leading to the sea. They have a layer of freshwater, below which the water is salt. Blue holes are not confined to the Bahamas, but are found at a number of other locations.