It's a fantasy role-playing game.

DM: You are Fjord, a half-orc warlock . You are the member of the famous group of adventurers. Previously, When filled with blood and given the proper ritual, it's used to acquire auguries, various bits of divination, and communication across the Divine Gate with the Scaled Tyrant. Caleb tells Nott it's of no use to them, but puts it in his satchel.
DM: You pull under, and you're uncertain as to where it'll end, and that brief little peaking fear in the back of your mind, flashes and memories of being under the ocean, of the shipwreck, of looking up and not knowing which direction was up. That gives you this brief surge of vertigo. You stop for a second, and a couple bubbles escape. Go ahead and make a constitution saving throw.
Player: 21.
DM: 21. You push through, continue, swim across, swim. "Learn." You come up, and all of a sudden you can see now, it begins to rise up once more, and you begin to swim up, pull across the front, and then suddenly your fingers break water, but you do so carefully. Take a breath slowly as your head emerges from the dark water in what is a large, darkened chamber. Make a perception check with disadvantage.
Player: Six.
DM: You see where this 15, 20-foot pool that you've emerged from opens up into this chamber that is filled with low water and a number of divots and pools within. The bars of land are limited and thin. You see from the peripheral of your vision, before it ends at that point, roughly five or six more of these creatures. You see a very faint glow in the distance, a number of objects that are piled and shoved into a central place. You see elements, old pieces of stone structures that have fallen in. This chamber inside here had a built or constructed purpose long ago, and since has crumbled as the earth has shifted. That's as much as you can make out.
Player: I'll take a big breath, blow it all the way out. (inhales deeply) Go back under, push back.
DM: Okay, cool. Making your way through, now you know the path is there, the confidence because you know there is air eventually helps you push through. You make it halfway through--
Player: What's the depth like as I'm going through?
DM: It's about 15, 20 feet.
Player: So I can see the bottom?
DM: You can see the bottom, yeah.
Player: That's not so bad.
DM: You do manage to come out the other side, emerge, and take your breath. Now that you've been through it once, and managed to go through it a little more confidently, you didn't have to worry about holding your breath.
