What do you mean, the old couple across the lake are perfectly lit up. I'd crop the photo though...the blown out people in the foreground are a bit distracting.
Klein may have a point. If your metering was on spot or center-wieghted and your focus point was set to the middle, then the flash may have tried to send the right exposure accross the pond. But, I haven't tried the 3200 watt 580ex yet, so I'm not quite sure how it responds.