Droop Snoot - at the very least they should have had the rad alt audio warning set at a no-go below height to stop them flying into the sea.
Ideally for a low level flypast over a smooth sea they should have had the rad alt height hold engaged - depends on how impromptu the flypast was.
Unlikely to have had any other AP upper modes engaged though.
At high speed and very low level any inadvertent selection or deselection of AP modes could cause a sufficient departure from the desired flight path to end in a scare or even water contact.