I was one of those who initially thought that the river was actually a full blown ocean, and that we landed south of Baldur's Gate rather than somewhere east. Although I think it has to do with the coloration of the sand looking quite a bit like Fort Joy, which WAS surrounded by an ocean, more than anything else. I forget, weren't there shells on the beach too, or do I have that confused with the DOS games?
I had a thought while making this post, though. It'd be a bit neat if NPCs reacted to thrown objects in their environment, so there should be shiny rocks around the river beach that you can use for this purpose. Precedence already exists with that rubber ball and the dog, after all.
Sure, this encourages ambush/alpha strike behavior more than before by potentially luring enemies into a situation that they wouldn't normally find themselves in, but at least it's interesting.