Mr Rosson slept in the cabin for the return trip, emerging about 7pm to go the toilet.
"He seemed fine to Mr Hanna, and as he headed out of the cabin to urinate over the side Mr Hanna warned him to be careful; Mr Rosson said he'd be right,'' Coroner na Nagara found.
But Mr Hanna felt the boat shift and went outside to see what happened.
Mr Rosson was not on the boat so Mr Hanna turned it around. He saw Mr Rosson in the water with his hand raised and threw a rope to him, but missed.
By the time he turned around the boat again to attempt to reach him for the second time, Mr Rosson was face down in the water and unresponsive.
"Mr Hanna was able to grab his friend, but could not lift him on the boat. He secured him with rope so he would not be lost and radioed for help,'' Coroner na Nagara said.
A toxicology analysis, taken after Mr Rosson died, showed he was almost three times the legal blood alcohol limit for drivers.
Mr Rosson was not a strong swimmer and was not wearing a lifejacket.
Coroner na Nagara said had he been wearing a lifejacket it was highly likely his death would have been avoided.
"Equally, however, it is highly likely that had he not been intoxicated he would not have ended up in the water in the first place, and he may also have been in a position to assist with rescue.''
Mr Rosson's level of intoxication was the primary cause for his death, Coroner na Nagara said.
His death served as another reminder of the very real risk of being drunk on boats, she said.
