In Dagny's pictures, you'll find one of a location on the ship that doesn't look worthy of a photo; if you don't recognize the spot, pay attention to Nancy's comment.
In the messages, there's one from Magnus referring to some sort of hiding place in the captain's bunk. You might want to take a picture of that message with Nancy's phone.
The rest of the messages will give you some information about Dagny's personality, but otherwise aren't useful. So what now?
Hint 7
Well, actually, aside from asking the folks in town what they know, there's nothing else you can do to find Dagny. In fact, she'll show up on her own again when you least expect it --
Hint 8
-- after you solve the sketches and go back to the lighthouse, in fact.