They can't use a flare gun or even leave the submersible because they were sealed in from the outside. They are likely on the surface because the banging was picked up by buoys, but the search area is so large that it's unlikely they'll be found. Also the banging is part of the search and rescue process as they bang every half hour.
Frightening.
The issue I imagine is the stress and horror of being trapped in a coffin for several days beforehand.