Stewart and Sonja Goodsir were on the water heading home after a day's fishing when they noticed the smoke.
At first it was "wispy" and floated through the cabin - but less than four minutes later they were "leaping off the sides" as their 13m launch erupted into flames.
"We just got off and we felt a burst of heat. We had only just gotten away from it when we first saw the flames," Mrs Goodsir said.
The Formula 4000L launch was at Crusoe Rock, halfway between Waiheke and Motuihe islands, when the smoke first became visible just after 6pm on Wednesday.
"I was up the top and he was emptying the holding tank because he wanted to do all that before we hit the rough water across the straight. We got the accelerator to give it a bit more juice and then it started smoking out the back."
Mr Goodsir turned it off and went below to investigate.
"He said, 'It's not right, it doesn't look good in here,' and handed me a lifejacket," Mrs Goodsir said.
The couple were well prepared and both had small fire extinguishers but could not see any flames at that point. "The smoke was wispy smoke but it gathered momentum quickly."
They scrambled into the dinghy that was attached to the stern.
"The whole exercise from kickoff to leaping off the sides would only have been four minutes."
It was devastating to watch the launch burn, Mrs Goodsir said.
They had spent the previous night on the boat in the marina and had been fishing in the Hauraki Gulf when they decided to head home.
Mr Goodsir was able to call the Coastguard for help before they abandoned ship and they were in the dinghy for only a couple of minutes when another boat arrived. It is still not clear what caused the fire.
Mr Goodsir yesterday retrieved the dinghy from where it had been towed back to shore and was in touch with his insurance company. He was not sure exactly where the launch went down but a Coastguard spokesman told the Herald it was about 300m from the shore.
Despite the loss, he said it could have been worse, while Mrs Goodsir was reflecting on the loss of their catch of the day.
"We had a lovely kahawai to be smoked when we got home. It was all smoked though, just not the right way."