There are a million scholarly theories about the meaning of the shark in Jaws.
Some see it as a metaphor for our limited understanding of nature—unstoppably powerful, unheeding of man, driven by instinct, terrible and majestic.
Others see him as a symbol for the then-recent Watergate scandal, representing the mortal danger that follows when elected leaders (we're looking at you, Mayor Vaughn) betray the society they're supposed to serve.
Comparisons have been drawn between the shark's underwater menace and the Japanese torpedoes like the one that destroyed Quint's USS Indianapolis in WWII. (source)
Some see the shark as the Cold War menace of "red" Communism seeking to destroy the America we love.
Freudian commentators consider the shark a phallic symbol demonstrating everything from the destructiveness of masculinity to the impotence of modern man to castration fears (source). Quint certainly gets castrated in the shark attack, along with losing most of his other parts.
And of course there's quite a large group that says, "Whattya talkin' about? The shark's scary enough just as a giant swimming monster that eats people!" (source)
Whatever you think, there's no question that if Jaws is a battle between good and evil, the shark definitely represents evil. Swimming, stalking, chomping, relentless evil that's even more terrifying because it strikes randomly.