I am locked in a tiny vessel with limited oxygen supply, how do I maximize my survival time?

If I found myself in a tiny underwater vessel, three kilometers underwater with only 12 hours of a 96-hour oxygen supply left and no electronics, this is what I would do:

First, I'd work to stay calm. Panic increases the breathing rate and therefore the consumption of oxygen. By focusing on my breath and slowing it down, I could maximize the use of the remaining oxygen. I would limit any physical activity to conserve energy and oxygen, and I would utilize any shallow, slow breathing techniques I know, like those from meditation or yoga. The aim would be to slow my heart rate and reduce the amount of oxygen my body needs.

To signal for help, I would utilize any available mechanical or manual noise-making device, striking it in a pattern to create an international distress signal - SOS in Morse code (...---...). Sound travels well in water and could be picked up by sonar systems in the vicinity.

If the vessel was equipped with a buoy or marker that can be released to the surface, I'd deploy it. This buoy might be detected visually or by radar.

Lastly, if any emergency external pyrotechnic devices were present, I would consider using them.

Remembering that survival is the ultimate goal, I would make every effort to conserve oxygen while sending out as many signals as I could, hoping that rescuers would reach me in time.