One of the cats was pregnant and an RSPCA manager says they were "left like rubbish".
Clare Wadham says she hopes the warning signs will make people think twice before entering the water.
Claire Wadham's son Cory died when his kayak capsized in a fen waterway last July, a coroner hears.
Firefighters advise residents to keep all windows and doors shut.
Cory Chilvers,18, had been missing for three days before his body was recovered, coroner hears.
Cory Chilvers, 18, is described as a "kind, loving, funny and thoughtful" son and big brother.
Norfolk Police say the family of an 18-year-old who has been missing since Sunday are receiving support.
The search for the 18-year-old is ongoing after he disappeared near King's Lynn at the weekend.
