Oceanus used a weapon to remove most of Mera’s powers. He used his hard-water powers to conquer Atlantis and turn Aquaman and Aqualad to mere servants. Mera agreed to become his Queen and they reigned together for a period.
Mera earned his trust enough to take his weapon in her hands … and then turned on him. Oceanus was able to bargain his escape at the cost of never returning to Atlantis. His powers remained intact.
In the events leading up to Final Crisis, it is shown in the Book of Destiny that shortly before Aquaman’s wedding to Mera, Oceanus was possessed by the ancient alchemist Megistus, in an attempt to kidnap Aqualad and use his powers to access the knowledge of the ancient Atlanteans. After being beaten by Aquaman and the Teen Titans, Oceanus is killed by Megistus so that he won’t reveal what he knows.
Oceanus shares several powers with Mera and other members of their race. Their world is covered entirely in water and their race are amphibians. Swimming is their equivalent to walking. Their main power is hydrokinesis. They have the ability to psionically increase the density of volumes of water and shaping them to create hard-water constructs. He can use them to cause blunt force damage or simply drown opponents. He has low-level telepathic powers, allowing him to communicate with others. He can not perceive the thoughts of individuals not broadcasting to him. His main weakness is common to all his race. Lead causes them to loose their hydrokinetic powers for the duration of their exposure to it