Two fishermen who survived two nights adrift off the South Australian coast had been counting down the seconds until they died.

Haiden Flanagan-Cox and Lee Eccles had ventured out on their half-cabin vessel at about 10pm (AEST) on Friday and were due to return at about midnight before forecast bad weather rolled in.

But the weather hit earlier than expected, the pair lost their communications gear, the engine started to fail and water started coming in over the back of the boat.

"It was just a big tail of events and one thing led to another," Mr Flanagan-Cox told the Nine Network on Monday.

"We just slowly watched the land drift away and saw some of the biggest waves we've ever seen."

The pair were adrift for two nights until a rescue helicopter spotted them Sunday morning in the Bay of Shoals off the north coast of Kangaroo Island.

"We never would have thought in a million years that we would have made it through that passageway, and through that surf, especially drifting," he said.

"The boat didn't take on massive amounts of water and we didn't capsize. I was just counting the seconds down thinking that's what was going to happen and we weren't coming back."

Mr Eccles said they were in survival mode the entire time.

"We weren't sure if we were going to make it back."

The pair thanked the sea rescue volunteers and police involved in the successful rescue.