Lol simply time yourself.
Go underwater count how many seconds past before yo character starts gasping for air.
You will then [know] exactly how long yo character can stay under water.
Best part about this is u will not need to depend on the game to know how long you can hold yo breath.
From that point foward simply start counting when you go deep sea diving

Btw i don't have the game feel free to tell me the magic number or i will just find out tonight when i get the game :]