They Couldn't Even get the Definition of League Correct

While you're correct that a league is a measure of distance, it is not necessarily a distance traveled, though that is what is the implication of the title of Jules Verne's novel. You could, however say 20,000 Leagues Under the Sea and mean 20,000 leagues of depth, though you probably never would as that would be 60K miles deep, we'd have to have a much bigger planet to have an ocean of that depth. So you're right and your wrong. They certainly use it wrong in this movie, implying that 'nemo goes deeper' is the meaning of the new title 30,000 Leagues... a trifling, but important, distinction.

As a writer and editor, I could see a mistake in the grammar of "30,000 Leagues Under the Sea"...under meaning a measure of depth as opposed to "30,000 Leagues Across the Floor of the Sea and a lot of the middle of the Ocean too".

English is a weird thing. Under does assume depth. Of course, one could argue the use of the word Sea in this title as well, as opposed to using Ocean.

I wonder if I should watch this film. I just kidded with someone about the film being 10,000 leagues deeper, but that was a joke. I didn't know they assumed the leagues were down and not across. But, it would be cool to see a film where Nemo drives right through the Earth and back out the other side and into space.

I don't get too down on movies like this. They are supposed to be fantasy. It isn't based on real events. I know there is a sense of applied science to it, but overall, I know I am not going to learn physics watching it. I should probably mute the TV so I don't absorb a lot of pseudoscience, but I feel it is so B movie, I'll probably end up liking it because it's so bad. At least for one watch.