The Argentine striker, 28, 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. Sounding conversational and jokey, he said he was "so scared".
Media in Argentina reported he said: "I'm on a plane that looks like it's going to fall apart."
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: "There is as yet no trace today of the missing aircraft. The search is ongoing and a decision whether to continue will be taken later today."
Officers are also "reviewing satellite imagery and mobile phone data to see if they can be of any assistance in the search".
Sala was heading to the Welsh capital after signing for the Bluebirds from French club Nantes in a £15m deal.
The single-engine plane left Nantes, north-west France, at 19:15 on Monday and had been flying at 5,000ft (1,500m) when it contacted Jersey air traffic control requesting descent.
It lost contact while at 2,300ft (700m) and disappeared off radar near the Casquets lighthouse, infamous among mariners as the site of many shipwrecks, eight miles (13km) north-west of Alderney.
Mr Fitzgerald said: "Sadly, I really don't think, personally, there is any hope. At this time of year the conditions out there are pretty horrendous if you are actually in the water."
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