The Alpha Centaurians were aliens from a planet where life had evolved on land before it migrated seawards. When their planet began losing its water due to vapor dissipating into space, they built a fleet of ships to steal water from other planets.

The Alpha Centauri trinary star system boasts other worlds with
intelligent life. Alpha Centaurians once came to Earth to steal an ocean but were repulsed by the Sub-Mariner.

also, if you look at the top frame from the superbowl trailer, it
appears leviathan is in this movie as well.