SANTA BARBARA, Calif. — Marine biologist Kristy Finstad first put on a dive tank at the age of nine.

The 41-year-old co-owner of Worldwide Diving Adventures was leading the scuba tour off Southern California when fire engulfed the dive boat, Conception, early Monday with 39 people on board, including six crew members.

Her brother Brett Harmeling of Houston said Tuesday that Finstad was among dozens missing and presumed dead off Santa Cruz island, part of California's Channel Islands. Harmeling thanked everyone in a post on his Facebook page for their "unconditional love and support during this incredibly tragic time."

"No final word on my sister Kristy; however, it is likely she has transitioned to be with the good Lord," he wrote.

Authorities say five crew members were rescued and 25 deaths have been confirmed so far, but no identities have been made public.

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Harmeling described his sister to the Los Angeles Times as extremely strong-willed and adventurous.

"If there was a one percent chance of her making it, she would have made it," Harmeling, 31, said.

Finstad had done thousands of dives, including hundreds to the Channel Islands, where she first swam with her father as a toddler. She first dove with a tank off Mexico, according to her company's website.

Her mother founded the diving company in the 1970s.

Finstad studied damselfish and corals in the Tahitian Islands, dove for black pearls in the French Polynesian Tuamotus Islands and counted salmonids for the city of Santa Cruz, where she lived. She also did research for the Australian Institute of Marine Science and wrote a restoration guidebook for the California Coastal Commission.

"My mission is to inspire appreciation for our underwater world," she wrote on her company's website.

She and her husband had just returned from sailing across the South Pacific. It was part of their 10-year plan that started on the back of a napkin in 2006.

They left from the Channel Islands in 2015 on the journey, and planned to return in 2018. Before they set off, Finstad wrote: "Our new boss is King Neptune: when it blows we hunker down; when there's surf we paddle in; when there's rain we wash up. In terms of time-lines, the best we can do is point in the right direction."