The warning from the state's Department of Land and Natural Resources came Wednesday amid reports of large sharks in nearshore waters. On Tuesday, a man was bitten by a shark off Oahu.
The 40-year-old man was hospitalized in stable condition with injuries to his right leg and fingers. He was attacked while sitting on his surfboard 300 yards from shore.
It was the second shark attack in Hawaii this year. A woman was bitten July 25 while snorkeling off Oahu.
Department Chairwoman Laura Thielen says people shouldn't enter the water were shark warning signs are posted. She says caution should be used even where no signs are posted.