The boat was on a Labor Day weekend diving excursion when the incident happened.
RELATED: Search suspended for possible survivors in deadly boat fire near Santa Cruz Island, Coast Guard says
Many of the 39 people aboard the 75-foot Conception were thought to be sleeping below deck when the fire broke out about 3:15 a.m. Monday.
The vessel was anchored about 20 yards off Santa Cruz Island when the charter became engulfed in flames, said Mike Eliason, a spokesman for the Santa Barbara County Fire Department.