If these are wild swans, then there isn't much you can do as the swans are going to choose the nesting site and you will not be able to protect the site. If you disturb the site or try pen the swans, you will not be successful.

Besides predators as being the cause of the cygnet deaths, we would propose another possibility. The salt water. This can kill older swans and especially young swans as the Mute Swans do not have a filter in their system to filter the salt from the water. This can lead to dehydration and any other number of health issues. If there is no fresh water in the area, this would be extremely detrimental to the swans. The Regal Swan