For Black Manta, he is a ruthless human treasure hunter who despises Aquaman for foiling his operations, although their conflict became more personal during The New 52.
So for Aquaman, using Ocean Master and Black Manta as the adversaries is a wise choice, not just because of how important they are to the Aquaman mythology, but because of the different ways they threaten Arthur Curry.
For Ocean Master, he is determined to seize the Atlantean throne from his half-brother, and will do whatever nefarious deed is necessary to make that happen.
I imagine their dynamic will be similar to the one T’Challa and M’Baku will have in the upcoming Black Panther movie.