Finally saw this. And my reaction is, it was fine.
I enjoyed it. It looked good. The water/aquatic stuff was at least a little different from the first movie.
But, this is not a movie that demands to be seen again. Not in the same way the first one did, because the visual impact was so much greater and more novel then. The effects in this movie are mostly of the same variety that people are now very used to. I read how they had to invent new technologies for the underwater scenes, but even if that's true, they didn't stand out like "omg, this is something we have never seen before," regardless of how hard they might have been to actually do.
Don't get me wrong, it looked quite good, but not that much different from the 1st one.
And the plot was the weakest part for me. Definitely felt a little paint-by-numbers. And just not all that significant? I know they already shot the next one, so maybe this one is just suffering from middle child syndrome. More set up than pay off in many ways.
Call it a B? I guess?