Genre: Action; adventure; fantasy
Minor spoilers - just assume this is the case every time.
In short - it was fine. Mediocre might be an equally suitable word.
Things that I liked were the fight choreography. I'm not very familiar with what trident combat would look like in the hands of superhumans or in the more buoyant environment of the underwater kingdoms, but it looked good and was exciting to watch.
Things that I didn't like were that the movie felt very busy, trying to cram too many plot points into an already over-extended and puffy movie. I didn't like how much of it backgrounds and set pieces were CGI. I especially didn't like how many times clunky and exposition heavy dialogues were interrupted by conveniently timed/placed explosions. On that note the Black Manta antagonist was too quickly introduced and shuffled off, and the alleged moral crisis / wound which should have been afflciting our hero was only touched on here and there.
It was fine. If you found yourself watching it by happy accident or convenience you'd live but I don't think this is something a movie-goer should seek out.