I surveyed the nests again this evening after a week away. It looks highly unlikely there will be any breeding success this year. Nests 3 & 4 are now abandoned with no sign of eggs. There was one bird hanging around at the third raft (nest #5) but there has been very little nest building in the past week unless it has fallen apart and is being rebuilt again.
I have no idea what interactions with humans, pets or wildlife may have caused the nest failures, so I'd be interested in any observations others may have. I know boat waves have been a problem, but they wouldn't appear to be the cause for the last failures at nests 3 & 4 since the nests are still intact.
BB