If this movie deals with a brewing conflict between the land dwellers and Atlanteans, Ocean Master could hire Black Manta to orchestrate attacks on military (or even civilian) vessels at sea - framing Atlantis as the responsible party. This, of course, incites fear in the world's nations and builds pressure for their leaders to take action against this growing threat. At the same time, Manta could attack Arthur and his loved ones, with the intention of deluding the people of Atlantis into thinking the land dwellers are attempting to assassinate their leadership.
All part of Ocean Master's plans to usurp the throne from Arthur and incite war - while building up Manta as a credible, deadly threat of his own.