It is an underwater cave filled with sleepy Great White sharks
Spy Kids: Special Edition
The Spy Kids are in the cave of sleeping Great White Sharks, and Juni has to pee right now, but Carmen warns him that sharks only sleep in clear water, and if he does, it'll wake all the sharks up. A Great White Shark wakes up and chases them, with its mouth open. At the last moment the kids get away and shut the door behind the Great Whites.