On a recent 4th of July... firing over a pond... I waited until a group of kayakers (some from our party & some neighbors) had cleared the area. We had already made the warning call when one of the kayakers (the next door neighbor), decides to paddle in front of the muzzle to watch... she should be quite familiar since she's seen this for the 10 years we've been doing it. I don't know how she got there so fast. It was very disturbing.
A re-enactor group I interact with has added a forward spotter... a spare member of the gun crew who stand off the the side (follows the crew only by their audible commands) but keeps a lookout for ignorant spectators wandering about... who might get in front of the muzzle. The Gun Captain gets an OK from the spotter before firing.