This idea is meant to make the game a little bit more difficult without going to the extreme of making it a single death restarts the whole game. Right now it feels close to right but a bit easy in my opinion. How about a new Stamina bar that the player has to keep track of when considering their actions underwater. When this is in the green, the players oxygen decreases normally with the time left in the air tank. However, if the player finds themselves too much in a hurry, in a stressful situation & low on health (such as when being attacked by creatures and swimming for their life), stamina can go into the red and he/she begins to use up oxygen at a faster rate, a similar effect to how oxygen tanks became less efficient at deeper depths without the rebreather mask in SN1. This would give players another incentive to keep an eye on their health and battery life of their tools so that it doesn’t begin to effect their stamina and therefore their oxygen time.
For those who like their game easier, maybe the stamina bar could be included in a difficulty level just above survival mode (survival +), but below the extreme difficulty one life setting.