Luke Owen, his family, and other cruise goers were aboard the Dorset Belle when 10 people got into trouble in the water.
It was later revealed that a 17-year-old boy and a 12-year-old girl was pronounced dead following the incident.
Since the incident, the Dorset Belle has been impounded by police at Cobb’s Quay Marina in Poole.
A man aged in his 40s was arrested on suspicion of manslaughter and has been released under investigation.
Luke, from Kingston, told the Mirror that the 4pm trip which lasted around 40 minutes was the first to head out since Monday afternoon after bad weather conditions.
The 35-year-old told the Mirror: “When we looked no one was on the beach and before we left everyone was on the beach. It was empty, everyone was just staring.
“I’m first thinking it was a shark attack, because I did hear there was a shark attack a few weeks back.”
After speaking with the boat operator, Luke was informed that a person had been pulled from the water and another was missing.
“As he’s told me that, we looked up to the sea and the 17-year-old was being pulled out. The ambulance came across and collected him.”
The dad-of-two said lifeguards had warned people about riptides and not to swim close to the pier.
The people on board didn’t know about the incident until they docked back at the pier and saw the beach had been evacuated, Luke added.
He told the Mirror: “Nothing really affects me but when it’s something like that it kind of hits you.
“Especially as I was with the family, my little one especially, I wanted to get out of there.
“My son’s 14 so he wanted to see what was going on so we looked briefly and police were saying for people to move.”
“To be honest, to me it sounds like a terrible tragedy. It was said quite a few times about the rip tides the day before. And the lifeguards were advising people not to get close to the pier.
“There was announcements over the course of the three days, and there are flags you have to stay within.”
Luke told the Mirror that he “prayed for the people” who died and his thoughts are with their families.