Zangua takes a drink of the salty potion and recalls his brief sojourn in the Plane of Elemental Water. He reaches for the mermaid's hand and she grasps it tightly. With his free hand, Zangua takes the hand of the next party member to drink, and so on.
Once the chain is formed, Farfar takes a step back. He starts to stammer what probably would have been a few words of encouragement or well-wishing, but the mermaid starts to crawl back toward the ocean, dragging the party along with her. As soon as she is able to submerge she begins to swim in full, her long and powerful tail propelling them along a downward angle to the east.
She soon accelerates to a tremendous speed, forcing you all to grip as tightly as you can to hold on, and levels off as her descent takes you to just a few meters above the ocean floor. The darkness is total here, but after a few moments the mermaid's skin begins to emit a curious glow. A phosphorescent tattoo consisting of labyrinthine spirals appears on her back.
The mermaid slows down and comes to a long, sliding stop in the soft seabed. Thick clouds of silt rise up, obscuring what little you can see from the glow she emits, and she begins to crawl along the ground for a few meters. Then, quite suddenly, she plunges headfirst into the silt.
Half-swimming, half-crawling, she pulls you through a long, narrow, twisting tunnel that descends, curves, and rises up again. Whatever mechanism the potion uses to allow you to breathe underwater is affected by the silt; it is harder to draw breath here.
Finally you emerge and the mermaid lets go of Zangua's hand. All you can see around you is darkness, the mermaid's skin only illuminates a few feet of space around you, and there is nothing to see but silt. She makes a few short, high-pitched cries and gestures, then begins to burrow back down into the silt, her duty apparently discharged.
Your move, players!