Water is always filling the room. For a long time, when the water gets too high, you clear the plughole, and the water flows back out.
At some point this stops working, and the drainpipe is irretrievably blocked.
It's fruitless to keep diving down to fiddle with it.
The water rises ever further.
Eventually, you're faced with a choice: drown, as the air runs out, or climb up the ladder starting at the top of the room and disappearing off into the mist.
You don't know what's there, and you have to leave everything behind, but, once you realise it's that or drowning, the choice is clear.
You climb the ladder into the higher level, and that's where you'll stay until you outgrow that one too.