What you say makes sense to us. You have to consider, though, that we are not the target audience for movies like this. They want a wider audience. If only divers watched these types of movies, then I doubt they'd make money.
The average movie goer assumes that a diver carries an oxygen tank which gives lets them breathe underwater. They have no clue about long that tank lasts, and that it is consumed at a faster rate at 47M than it would be at 10M. They may have heard of "the Bends" and that it means the ascent needs to be slow, but they have no idea what that looks like or really why. They won't know what an AL80 looks like compared to a Steel 120.
I do think a couple of tweaks to the story to make the science work wouldn't have hurt the story in the minds of the bulk of the audience, and may have made it more believable to the educated audience.