Recognize the warning and the return point
The first recorded dive explicitly warns the player to go back to the submarine before suffocating. The yellow submarine remains visible in the upper part of the water. The turtle swims into it at 1:25 and the expedition ends.
Do not use ammunition as an air gauge
The portrait ring, the Full bag label and the tool counter appear at once in the example. A tool can refill its ammunition while the oxygen warning remains. Returning is therefore still necessary even if a gun counter has recovered.
The loss-reduction tooltip gives the missing distinction
A later Shell Insurance II tooltip says it reduces the jellyfish and resources lost upon asphyxiating. Its current-to-next loss display is 70% to 45%, with a 7,500-shell cost. This supports a loss-reduction effect; the footage does not isolate rounding or test every possible cargo amount.
Read the oxygen upgrade itself
The early tree offers Deep Breath, while a later Oxygen Tank II tooltip describes an increase to base oxygen. Those are different upgrade effects. Match the node’s description to whether you want consumption reduction or capacity; this page does not infer a universal safe dive duration.