I felt that way with their comic book counterparts.
Black Mantis is the perfect foil for Aquaman in terms of combat. A mercenary with military combat training, knows how to steal what ever he wants (including prototype subs from secret military bases), and what he lacks in physical ability he makes up for in Tech.
Most of Ocean Master’s threat is political, and the last time he tried to take the throne, his efforts were laughable.
Here is a quick gist, largely paraphrases to save time
Ocean Master: Brother I Want you to abdicate the throne
Aquaman: Sure, explain to the people of Atlantis how you are their new King, Then explain to all the sea life how you are their new king, then explain the whole thing to Mera.
And if all 3 agree, then I will step down.
Ocean master: Wait you said Mera, never mind then I want to live.
I am kinda man they really, really simplify Mera’s backstory and gave her a major nerf.
She more of Arthur’s equal than just his queen, she his equal as she can also hold her own against like likes of Superman