Water Walking is, primarily, a chakra control exercise. Aside from that BIG benefit, however, it should also help us at fighting on unsteady ground. Like sand./QUOTE]
Yeah. I mean, its whole introduction in the series is "this helps your chakra control". Sure, it increases mobility, but the initial narrative reason it comes into play is to help Naruto increase his (kinda terrible) chakra control. After that, I'd agree with
@Yog about utility on sand, or mud.
Plus, Konoha's in the middle of a forest, and we've seen there are rivers and the like.
Even more plus, I bet making WW better will boost Tree-Walking, which is also relevant.