Kiwi round-the-world-veteran Mike Quilter said there could only be grave fears for British sailor John Fisher, after he fell from the boat Scallywag during the Volvo Ocean Race.
Quilter, who took part in five of the yachting epics, said survival time in the water might be as little as 20 minutes.
Fisher — who was wearing survival gear — is reported to have been missing since the early hours of Tuesday morning (NZ time) after he went overboard from the Team Sun Hung Ka boat about 1400 nautical miles west of Cape Horn.
Water temperature was 9C with 35-knot westerly winds, and "accompanying sea state", an official report stated.
Quilter said that Fisher was either "not clipped on" or his harness had broken.
Quilter, who lives in Kerikeri, said crews he had been part of had barely suffered an injury during five races but the dangers were still there as boats strove for victory in extreme conditions.
"It's a toss up sometimes ... good seamanship versus a desire to win," he said.
"Sometimes I would think this is crazy ... we've got to slow down. But if you don't put up the big sails you finish last. You have competitive people who want to win."
He said the water could hit sailors "like a wall".
When asked about Fisher's survival chances, Quilter thought they were probably "nil". It was an impossible task in the conditions to turn a boat around quickly enough even though the alarm would have been raised immediately.
"In a heartbeat you are miles away from where you want to get back to," he said.
An official statement said that because of gale force conditions, it was not an option to divert any of the other six yachts, which were at least 200 miles east and downwind of Scallywag.
Quilter said: "Obviously your heart goes out to those involved. You can only imagine what he [Fisher] would have been feeling, as the little light [of Scallywag] quickly faded into the distance."
Quilter said he did not like the Volvo Ocean 65 class of boats, because they sat so low in the water. He said it would add to the discomfort and possible even the danger because the sailors would be constantly wet.
Teams are on day nine of Leg 7, a 12,200km race from Auckland to Itajaí, Brazil, but Team Sun Hung Ka/Scallywag was some distance from the other six teams when the man fell overboard about 2250km west of Cape Horn.
A ship about 400 nautical miles away has been diverted to assist.
Fisher, 48, who lives in Adelaide, is in his first Volvo Ocean Race after previously competing in the Sydney to Hobart race.
A person died in an earlier leg of the race after Vestas 11th Hour Racing struck a fishing boat about 40km from the finish line of Leg 4 in Hong Kong.