I just watched the CNN upload "Dive boat owner gives insight into deadly California fire" in YouTube.
The owner confirmed that the boat had fire extinguishers and even a fire hose. He also confirmed there was a separate hatch, and that the bunk room is never locked.
As In the Details (pretty sure) said earlier, this must have been one of those "flash" type fires or even something like an explosion. Perhaps some sort of gas or fumes that leaked and accumulated before being sparked.
In one of the photos of the interior of the bunk area, you can clearly see a sign that says "Emergency Exit" with an arrow and what looked like instructions. The probably gave everyone instructions on how to use it. They passed so many Coast Guard inspections previously, it sounds like just a tragic situation.
That said, I never would have gone on an overnight cruise in those kinds of sleeping/boating accommodations. That's just me. Chicken - yes.
I feel so badly for the crew members who escaped. They must be very traumatized to have witnessed it so closely and been unable to do anything to rescue their friends and customers. One person interviewed on the boat that rescued them said that some of them kept swimming back to the flaming wreckage to see if they could rescue anyone. I hope they get some counseling. They'll be facing some terrible survivor's guilt and will need it.
Prayers and condolences for the survivors, family and friends. Terrible tragedy.