The fact that he apparently has annually come to the aid of a surface city with food implies he must have some measure of authority under the sea.
What I'm interested about is that they've mentioned that the Mother Blxes were all hidden and buried by Atlanteans, Amazonians, and humans, and how that might impact his story. The Amazonians likely have some veterans from that time still living, so it's not hard to see Them being fully on board with Batman's preparations, and Batman is brininging up the human end with Flash and Cyborg while trying to resurrect an adopted human in Superman. Aquaman's refusal to join may be a sign of his culture's growing isolationism that doesn't believe the story at all.