He wrote, "Please pray for my sister Kristy!! She was leading a dive trip on this boat."
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Kristy Finstad is listed online as a diving instructor, marine biologist and co-owner of Worldwide Diving Adventures out of Santa Cruz.
Her biography says she also previously counted salmonids for the City of Santa Cruz.
Worldwide Diving Adventures advertised a Labor Day Weekend dive-trip to the Northern Channel Islands, detailing the group would be sleeping on what's called a live-aboard.
Officials announced passengers were below-deck, likely sound asleep when the ship caught fire early Monday morning.
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According to the company website, the group was scheduled to return on Monday afternoon.
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Audio from the early morning mayday call between the Coast Guard and vessel uncovered crucial questions like the potential of locked doors, questions surrounding firefighting gear and extinguishers, and more. Only the Coast Guard broadcasts can be heard, not the responses.
At one point, the Coast Guard is heard saying, "Completely engulfed boat. We're not going to make an attempt with our pumps to put it out. Main objective is to look for victims."
Back in Santa Cruz, Finstad's neighbors spoke with ABC7 News off-camera. They said they're aware she and her husband, Dan Chua, are dive instructors.
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Many residents shared their concern over the tragic developments, and their hope the family would receive answers soon.
In follow-up public Facebook posts, Finstad's brother revealed Chua was leading a separate dive trip in Costa Rica.