They are playing 5th-edition, as per the reddit thread they linked. The 5e rules state:
A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier.
Your character can hold its breath for 7 (1 + 6) minutes, as its Constitution modifier is +6. Thus, your character can hold their breath for 420 seconds. Your DM is free to use their own rules, if they wish.
Note: As you stated you are playing a lizardfolk, you can hold your breath for up to 15 minutes at a time (this is part of your traits).
I'd like to point out that holding one's breath while actually doing something, like swimming, would severely limit how long you can go without breathing. Also, those pearl divers mentioned are practically naked and unencumbered. A typical adventurer that goes down with the ship would have to shed any armor (including wizard's robes) and drop weapons and backpacks just to be able to move or reach the surface. As part of my lifeguard training, I had to jump in the pool fully clothed and learn to disrobe while treading water, then make temporary flotation devices out of articles of clothing. It was exhausting.