The States police is investigating after a rude symbol, foul language and other graffiti was sprayed in yellow and blue paint across an extensive area at the weekend.
Sean Kinsella, from Absolute Adventures, discovered the vandalism as he was about to start beach activities with a party of children at about 10 am on Sunday.
The watersports instructor also found litter from a barbecue.
It is the second time in just over a week that Mr Kinsella has complained about the state the area has been left in by beachgoers.
Inspector David Webster, from the States police, has urged anyone with information about the party on Saturday to come forward.
Anyone with information is asked to call the police on 612612 or Crimestoppers on 0800 555 111.
