Nathan drake is not believable.
That's right, I said it. Gaming's every-man is just not that at all. He's athletic and nimble in ways that would make Ryu Hayabusa jealous, He's got better aim with a gun than myself or anybody else I know, and he's WAY too cool with killing people.
I don't know what to say about the concept of the player "saving" Laura, but I found her to be a much more believable character than Nathan Drake. Hell, I was in the same situations she was in, I'd probably react the same way she does. I'm not athletic, I'm not heroic, and if the ground underneath me gave way and I ended up getting swept up in a raging river, I might in fact scream like a girl too.
I don't know if the designers had this in mind, but I think this is a situation where a character will actually start out where I'm at, and then grow with me. This never happens. Even in RPGs starring a child lead, the character you play as is typically already an accomplished sword fighter or gifted in magic, or able to talk to animals or some other such nonsense. That's not the case here. I think the game will begin with Laura being a character much like the person actually playing the game, then evolve into the Laura we remember from the classic games. As long as she eventually gets there, I'm totally cool with her starting off weak, fragile, and frail. We ALL start there, it's what she becomes in the end that matters.
Of course, we'll have to see the end result to really judge that.
