Hi Ginny
First, we would raise the nesting area so the tidal wave cannot affect the nest. You need to ensure that the soil under the nesting site is fully packed for each layer you fill. Even then, a strong tide may still flood the nest.
Second, when you see the swans looking to nest and searching for nesting material, show and provide them with cut grass or straw and place it higher on your property. There is a chance that they may make a nest where you place the nesting material. However, there is no guarantee that they will not return to the old nest because swans continually return to the same nest. They possibly do this because of familiarity with the area, presence of good food resources, protection from predators, shelter from inclement weather etc. But, if you can provide nesting material near the old nest on higher ground you may still have a chance of luring them to the new nesting material. The Regal Swan