Parents need to know that this movie has some strong sailor language and some sexual references that can get crude. Characters drink and smoke a great deal. For most parents, the primary concern will be the scariness and sadness of the movie. It is very intense and many characters are killed. Parents should be willing to give kids deniability ("I really want to see it but my parents won't let me!") if they sense that the kids do not want to go.
I agree: "scary and sad are the main concerns here." That's because I thought this would be good and it was so scary, disappointing, and sad about how "so-so" it was. But it's very intense.
My rating: PG-13 for frequent and realistic intense disaster sequences, innuendo including double entendres, brief alcohol use and language.