"When we got them on board they could hardly stand up. They were exhausted and in shock of seeing their friend go under the water.
"They were very lucky, we would not have spotted them if we had not heard them."
Ross estimated his boat arrived only moments after William disappeared under the water as he tried to swim the 500m to shore.
"We were pretty much on the incident as it happened."
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He said they took the four men back to shore where an ambulance and Coastguard were waiting and then went back out to try and find William.
"My sympathy and condolences go out to the family.
"At the end of the day, a bunch of young guys were having fun on the water and made the mistake of thinking they could go out deeper in an overloaded boat with no life jackets.
"It's easy enough to do ... but it's a simple message to always carry life jackets."
Ross praised the rescue services who attended the emergency.
"The Coastguard's response was fantastic. They were there in 15 minutes."