The term has since been applied to whales, with our word "cetacean" obviously deriving from the Greek. There are several species of whale inhabiting the Mediterranean, and most of them are the toothed kind rather than the baleen variety (the fin whale is an exception), so I suppose they could theoretically eat a person. This also calls to mind the creature that swallowed Jonah, referred to as a "great fish" in the Hebrew, but popularly called a whale. I have no idea whether the author of Jonah would have known that whales weren't fish, or whether he had any particular species in mind for Jonah's captor.
The constellation Cetus is found among several others from the story of Perseus, so it's presumably supposed to be seen as the one slain by the hero. The group of stars is now typically called "the whale," although as with with most constellations, it really doesn't look like much of anything.
Still, its association with a sea monster is an old one, with ancient Babylonian astronomers identifying it with Tiamat.
The kraken actually comes from Norwegian folklore, and there are a few different indications as to what it might be. It appears that the earliest known mentions of the monster are from the eighteenth century, and refer to the kraken as a crab-like monster the size of an island, which eats smaller fish, but also nurtures their growth with its excrement. In popular culture, however, the kraken is generally thought of as a gigantic octopus, possibly based on the real giant squid. As with most mythical sea monsters, encounters with them are incredibly dangerous and usually fatal.
While I don't know about the new Clash film, the monster in the old one doesn't at all resemble an octopus or even a crab, so I guess they just used the name "kraken" because it's more familiar to modern audiences than "cetus."