If they experienced an emergency underwater and were not able to surface, I'd fear for the absolute worst.

People can survive in sunken submarines long enough to be rescued, but those are typically large submarines with multiple rooms that they can seal themselves off in. I doubt a tourist submarine, or really any submarine going to this depth, has multiple compartments. Many military submarines have a test depth of around ~1,500ft or less & a crush depth of around ~2,500ft or less. Titanic sits at 12,500ft. It takes specialized subs to get to this depth, and they're typically very small, one cramped compartment, vessels.

Even if they sank but did not suffer a leak, it's unlikely there would an air supply long enough to support a rescue.

If they sank, a rescue seems very unlikely. Nonetheless, I hope for the best.

Regardless, for a sub like this to just go missing, likely means something catastrophic happened. At those pressures, the individuals likely died instantly and felt no pain.