A 19-year old man was hospitalised after finding himself in difficulty while swimming in St Paul's Bay on Sunday evening.
Police said the man, a Romanian national living in Qawra, had been swimming close to Triq Dawret il-Gżejjer.
He was pulled from the water and given first aid by other swimmers.
He was then taken to Mater Dei Hospital for treatment, where his condition was certified as serious.
A magisterial inquiry has been appointed and police investigations are ongoing.