https://m.youtube.com/watch?v=QlRp__ZB0Zg
So I was watching this gameplay demo of Injustice 2 and unfortunately it seems like Black Manta is just part of the environment. He appears at 1:46. We also get a look at the new gear system.

I do not like the gear system at all.
If I ever get a PS4 and Injustice 2, I'd hope that it'd be a system you could disable (Though I doubt random fights online would have it disabled)

According to Maximilian DOOD the movement is faster and less stiff...

I hope he's right.