That's what happened to 28-year-old Ian Cole of Michigan last Saturday. The boat had accidentally left without him 15 minutes earlier.
"Your adrenaline kicks up. I had a little bit of a panic," said Cole. "So at that point, I was trying to regain my bearings."
Cole was able to swim to another tour boat that was still in the area.
A staff member who conducted a head count aboard Cole's tour boat broke the rules by failing to get Cole's signature to confirm he was on board before it left, Association of Marine Park Tourism Operators executive officer Col McKenzie said. The staffer was fired.
Cole's father, Jim Cole, said his son grew up in Farmington Hills, Michigan but has been living and working in Australia for about a year and plans to return to the U.S. on July 4.
Australian officials are investigating the incident.
The Associated Press contributed to this report.
