I have to day after reading your review I agree with you on the fact that as far as the overall plot this movie has little to add. There are small bites of info we learn about the monsters as you said they came from above, and they can't swim but other than that this film seems very much like it is just a study guide to the test as in things to remember. Which is strange considering Krasinski never intended for there to be a sequel. While writing this response however it dawned on me that the first movie was more about the daughter and mother and this one was more about the son having his own things to overcome.