The good news is I managed to find the treasure, tho I was starting to think it is an impossible task, as the treasure is undetectable by sonar and you have to use your eyes. It turned out you can recognize it by it being the only emitter of particles in the game world, though from distance it is hard to distinguish it from dark background. It is useful to invest in Used to Darkness perk which improves your night vision. The treasure is somewhere between depth level 44 and ???

The bad news is nothing happened when I found the treasure, the sequence didn't fire, so I fixed the problem and uploaded the fixed version, but anyone who has downloaded the game prior to six hours ago should re-download it again, in case they want to have a version of game with working winning condition. On the bright side, the remaining two endings (having hull crushed or running out of power) work perfectly.

Btw this screenshot has nothing to do with me finding treasure.