This is a case where Charlie Brown's responsible for his own disappointment, to some extent. It's his pool, right? That's why he filled it? I guess he doesn't want to throw his friends out when they're having such a good time. (Schroeder especially is getting into it, although I doubt he really needs to hold his nose as he jumps in.)
Why is it necessary for Charlie Brown to empty the whole pool just because Snoopy is sitting in it?
As a kid, I'm not entirely sure why, I loved the picture of Snoopy sitting, smiling, looking out of the pool at Charlie Brown. It's something about his profile, or his attitude maybe. Or maybe it's just really cute.
The shading on the underside of the pool as CB empties it is an unusual touch for Peanuts.