Revealing mistake: When Shelby's corpse shows up in a Ben Gardner like jump scare, you can tell that the hair is not attached after they made a mold of Harry Grant's chest and face and edited it.
Continuity mistake: After the big shark dies Mike and the blonde girl are calling the dolphins, in one shot it's dawn and it's dark but suddenly in the very next shot it's completely daylight.
Revealing mistake: When the shark bites the fish in half just before the opening titles you can see the back of the fish tank the shot was being filmed in.
Continuity mistake: When Mike is in the water doing the welding on the tunnel patch he is wearing a full face mask with the rebreather attached at middle, but when he and Kay are swimming away and go into the door back to the control room after being chased by Jaws, he is wearing just a regular snorkel mask with the rebreather in his mouth.
Continuity mistake: The small shark shrinks a lot when it's brought into the enclosure. If it were its original size, it would have stuck halfway out of the tank.
Continuity mistake: The size of the small shark decreases a lot when it is caught.
Answer: The lagoon was part of the theme park. Artificial with specific mammals inhabitants inside, warm and friendly.