OK, just got out now. It dragged a bit in the middle eight hours (started to get a bit self-indulgent exploring the undersea world, a la 2001 or TMP). Could've been shorter (the way of water = down my leg).
I can see why there are mixed reviews. I thought it was OK but obvs lacking the "gee whiz" factor of the original. But two buddies absolutely hated it and thought the storyline (such as it was) was drivel.
I actually didn't notice the plot holes but it obvs pays to focus on the spectacle and not expect too much brain food.
It would be a tall order to top the original's box office without the same 3D novelty effect.
And as mentioned earlier, there are many Cameron-isms from Aliens to The Abyss to Titanic and even Terminator.