Boat owners are now being urged to check their vessels following the incidents, which happened between 2pm and 11pm on Monday.
The States police said a man and a woman, both about 5ft 8ins tall, were seen in the area at the time. The man had light-coloured hair and was wearing a khaki T-shirt and black and white shorts. The woman was wearing a pink jacket.
Two people – a 33-year-old man and a 41-year-old woman – have been arrested. Anyone with information is being asked to phone the States police on 612612 or Crimestoppers on 0800 555 111.