No. The Leviathan is a mystical creature of Hebrew and Christian faith, representing both a demon and a seafaring god. The Kraken is a legend derived from sightings of giant sea monsters, such as the giant squid. Check the links below for detailed descriptions of both the Kraken and Leviathan.
The kraken
A Behemoth is a land animal. The Leviathan is a sea creature. Therefore the Leviathan has the possibility of being the largest.
maelstrom or leviathan charybdis
As the Kraken is a legendary creature, and some doubt it has ever existed at all, it's almost impossible to know.
As the kraken is a legendary creature out of lore, it can be argued that no one knows, as no one has studied it.
The Kraken
The Leviathan is a mythological sea creature mentioned in various religious texts and folklore. There is no scientific evidence to suggest that a real-life Leviathan exists. It is believed to be a symbolic representation of chaos or a powerful force rather than a literal creature.
The kraken is a legendary sea monster in folklore known for its size and strength. While it is not real, it is often depicted as a dangerous creature in stories and myths.
The Kraken
The Kraken
Depictions of the kraken show a giant tentacled monster that is clearly some sort of cephalopod (octopus, squid, or cuttlefish). The closest creature to this known to science is the giant squid.
Well a Kraken is a mythological creature, so there never was a first sighting of something that didn't exist. But it was seen in Pirate's of the Caribbean ;)