There are suggestions in the movie that the sunken ship used by the sharks as a hideout is supposed to be the Titanic. The would be impossible, because the movie takes place along a tropical reef such as the one off Key West Florida. This is over 1000 miles away from the Titanic. Also, the sunken Titanic is 10,000 feet deep. The sharks and other fish in the movie can not swim that deep.
At Frankie's funeral, when the casket bearers put Frankie's body over the side he floats upwards, as a dead fish would do. A shark however, has no swim bladder and is heavier than water, so a shark - whether living or dead - will sink to the bottom when not in motion.