Continuity mistake: The size of the small shark decreases a lot when it is caught.
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.
Factual error: Although continually referred to by the workers at the park as "he, " the captive killer whale kept there is obviously female. You can tell by the shape and size of the fin.
Answer: The lagoon was part of the theme park. Artificial with specific mammals inhabitants inside, warm and friendly.