@KenV
If weather is bad, airshow flights are cancelled over water.
@Wensleydale
Planes at coastline airshows are below 500ft for sure! Bird strikes would seem to be more of an issue with prop aircraft, with jets the birds can't get away fast enough.
@Random Bloke
Beacons or indicators of some kind could be placed in the sea to give some indication to pilot, if they felt they needed it. They seem to be doing ok without though!