I gave this episode a 9.5/10 because it could have been really really bad and, knowing what the majority of the storyline was going to be, in a way I was expecting it to be bad. However I was very pleasantly surprised and I do think that it had a lot to do with Mark Harmon and Michael Weatherly's acting. Fantastic underwater scenes.
Still, like others have said the "Go back Daddy" was pretty much ugh! but that was the only real blight on an otherwise excellent episode.
I adore finding out about characters and their past and what I love about NCIS is that most of the time they are pretty subtle about it (Except for Gibbs and Jen's past relationship which got annoying very quickly - and also, though far more understandably, Gibbs' past with Shannon and Kelly)