State police spokeswoman Megan Peters said the 19-year-old man was sitting with his back to the water on the railing of the fishing vessel Confidence in Sitka when a large bull sea lion jumped out of the water and bit him on the backside, the Anchorage Daily News reported Tuesday.
Peters said the man suffered several large scratch marks but no puncture wounds from the animal's teeth.
"Thankfully he was wearing rain gear or it would likely have been worse," Peters said.
Authorities said the sea lion may have become used to people and may have been fed by humans in the past.
"We don't know if this particular sea lion had an experience in the past where it would associate people with food and perhaps that's why it was aggressive," said Julie Speegle, Alaska region spokeswoman for the National Marine Fisheries Service.