Coastguard volunteers went in for a closer look and found the man asleep. His boat appeared to have drifted for four hours, about seven kilometres off course, Shepherd said.
The man was "extremely intoxicated", he said.
"Police are appalled at this behaviour and would like to remind the public that the water can be an unforgiving place - conditions can change suddenly, and you need to take precautions to look after yourself and others.
"When out boating, don't drink - this could have easily ended in tragedy. This man's friends and family were understandably very concerned and a large effort has been undertaken by Police, Coastguard and volunteers to locate the man."
Police will be speaking to the man tomorrow, Shepherd said.