Jonah Payne; the first host of Dead Water was a sub diver who sold his talents to the Scavenger. He took the sub deep into the unknown areas of the ocean. When the sub became stranded he killed the two other men and was making his way back when he swam through the alien water. This substance took over Payne like a symbiote and taking in all the guilt and anger he felt, began to kill anyone whom Payne felt slighted during his life.
Now the possessing entity has returned and is terrorizing an oceanic study base deep within the amazon coast. The U.S. naval forces sent their Aquamarines to Arthur Curry's landside abode in order to help them deal with this latest resurgence of the creature.
Note: Dead Water first appears off panel in Aquaman #49 but does not appear fully formed until Aquaman #50 and is created by Dan Abnett, Vicente Cifuentes, and Brett Booth.