Los Angeles fire crews say they found the 35-foot cabin cruiser engulfed in flames shortly after 4 a.m. at the Port of Los Angeles' Berth 258.
Firefighters were able to extinguish the blaze in about 16 minutes.
After putting out the fire, crews discovered a 69-year-old woman deceased aboard the boat, which was still afloat.
The cause of the fire and the woman's death remain under investigation. The victim's name has not been released.
"She was a nice lady. She had a good heart," said Chris Girardea, who said he knew the victim.
No other injuries were reported. The cause of the fire remains under investigation.
City News Service contributed to this story.