When the heroes first arrive in the Nether Sea, read the following narration text:
After a few moments of disorientation, you assess your new surroundings. Monotonous black waves and alien skies stretch for as far as the eye can see. The stars are strange and unrecognizable. In stark contrast to the roiling oceans above, everything here is deathly silent. The smell of briny seawater has been replaced with a pervasive metallic odor. You can hear something sizzling against the hull of your ship.
If the heroes arrived here by sailing through the massive whirlpool of Charybdis, then they appear several miles in a random direction from the whirlpool’s location in the Nether Sea (area N1).