I think the underwater CGI for Aquaman is good. Most of the underwater CGI is created through wire work and green screen. Over 2/3rd of the movie is set to take place underwater, it would be really challenging but from the trailer I can tell that a lot of work has been put into the underwater scenes to make them look good.
Since most of the scenes of Aquaman are underwater, I believe much effort has been put into it unless the entire movie might end up being a disaster. Let's be hopeful and optimistic about the film.