UPDATE: Three fishermen missing since yesterday tied themselves with rope so they would not drift apart while waiting for help.
Details of the men's attempts to stay alive emerged after their dramatic rescue this morning.
The Westpac Lifesaver Helicopter found the three fishermen who had been missing for more than 15 hours after a trip went wrong overnight.
The trio were found in the water wearing life jackets off Stragglers Reef about 11am today.
The men - two aged in their 60s and one aged 79 - had been out on the water for about two hours yesterday when the boat sank within minutes.
The experienced fishermen had been fishing near Stragglers Reef.
They left Woodman Point about 5pm yesterday and were due back about 10pm.
They told Water Police they had an EPIRB but the boat went down so quickly that it did not activate.
"The Westpac Lifesaver Helicopter located the three men off the east side of Rottnest Island," Surf Life Saving WA said.
"Crew then directed police to the men."
The rescued men were returned to the mainland by Water Police and taken to Fremantle Hospital, where they are being treated for extreme hypothermia.
Rescuers said the men were close to death when they were found.
The men were wearing life jackets and tied themselves together with rope so they would not drift apart while waiting for help.
A search was launched this morning after the men were reported missing about 7.30am.
The 79-year-old skipper was one of three men aboard the boat.
It is unclear whether the men had told people exactly where they were headed yesterday but authorities started searching for them near Carnac Island because it was where they usually fished.
Eight vessels and two helicopters were involved in the search after the men failed to return to Woodman Point last night.