She fell from her stateroom balcony down to the deck below.
A traveler onboard says passengers are still waiting for the cruise line to tell them exactly what happened.
Carnival confirmed the death, saying the guest apparently went over the balcony of her stateroom last week.
The Carnival Firenze left April 26.
The fall was reported around 2 a.m. April 27, within the first few hours of the trip.
In a statement, Carnival said, "The female guest was traveling with family who alerted the ship's crew of the situation."
SEE ALSO: A suspected outbreak of the rare hantavirus on a cruise ship in the Atlantic Ocean kills 3 people
It goes on to say "as is customary, law enforcement was onboard while the ship was in Catalina Island on Monday to collect information. ... Carnival's Care Team is supporting the guest's family, and our thoughts and prayers are with them and their loved one."
One passenger says the FBI was among the law enforcement on the ship investigating.
The woman who died in that fall has not been officially identified.
Carnival says her family is off the ship and back home.
It's unclear if there was any "family dispute" the night she fell.
This was a four-night cruise.