While we're on the Two Towers film, Graham mentions being confused by "Aragorn getting thrown into the river', and asks 'What was that in aid of?'
My read of that scene is that Jackson is trying to develop a fall of a major character as a thematic element in each of the three films, at about the midway or two-thirds point. In the first film, we have Gandalf getting pulled down by the Balrog; the third film has an obvious candidate that I'll not explicitly mention, just to avoid potential spoilers -- but the second film didn't really have a good falling scene. So he wrote one in. Each of the three falls has similar aspects -- the fallen character lands in water (or, well, liquid, anyway), and the fall is redemptive in some fashion. Gandalf the Grey falls, and rises as Gandalf the White; "Strider" falls ("Strider" is the aspect of Aragorn avoiding his responsibility to be king) and
Aragorn, King of Gondor arises. Again, the redemption in the third fall is of a slightly different nature, but there are other elements of that fall that are different as well -- that one is the rock in the Zen garden that's just slightly out of kilter.
However, I also had about three beers before seeing the movie, so maybe I'm just full of crap.