The Argentine striker, Emiliano Sala and a pilot were on board the plane which lost contact off Alderney in the Channel Islands on Monday night.
Chief officer of Channel Islands Air Search, John Fitzgerald, said “even the most fit person” would only last a few hours in the water.
The search for the missing aircraft and its occupants resumed on Wednesday.
Sala reportedly sent a WhatsApp voice message to family, saying he was “so scared”.
Media in Argentina reported he said: “I’m on a plane that looks like it’s going to fall apart.”
As of 11:30 GMT on Wednesday, Guernsey Police said there were three planes and one helicopter in the air as they searched for traces of the Piper Malibu plane.
The force said it was also “reviewing satellite imagery and mobile phone data to see if they can be of any assistance in the search. So far today nothing spotted can be attributed to the missing plane”.
Sala was heading to the Welsh capital after signing for the Bluebirds from French club Nantes in a £15m deal.