One thing to be aware of is differing individual responses to cold water.  As an anecdote, I was paddling a cold northern river and my two friends dumped in the middle of a relatively long rapids.  Although they weren't in the water too terribly long, one paddler was deeply chilled and took literally hours to warm up.  The other shook off the water like a labrador and showed no ill effects whatsoever.  However, I agree with other posters that water temps in September aren't much of a concern.