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.