(I'll post pictures of the relevant characters here after 11.)
Past the Lanternfish's throat lay a nearly vertical waterfall. This was the Upper Esophagus.
Sephiroth piloted the biomimetic speedboat down the drop, activating its magnetic levitation system when he spotted the "islands" below.
Before the speedboat reached the "sea," its four mechanical legs hummed, suspending it above the surface. The water here looked deep blue.
This latest "swallow" by the Lanternfish had also brought in a vast amount of seawater, fish, and assorted debris.
Sephiroth noticed that the fish began thrashing and darting about after falling into the water.
They did not die immediately, and it looked as though they would survive for a while yet.
The water's color had looked abnormal even back in the mouth, but that had amounted to dilute hydrochloric acid at most.
This deep blue water probably had a higher acid concentration, though it likely wasn't anything too extreme.
Sephiroth's boat could hover, so that posed no problem. He couldn't really remember how the Straw Hat Pirates and the treasure hunters had dealt with the acid corrosion in the original story.
"..."
He steered the speedboat toward the islands, spreading his Mantra to encompass the entire "space."
This space should have been the Lanternfish's stomach, but it was large enough to hold several small islands with ease.
Clusters of intertidal rocks and the enormous skeletal remains of small Sea Kings pierced the surface, while waves washed over golden beaches.
Despite being inside a stomach, the surroundings were surprisingly bright. There was actually an enormous golden light source in the "dome" overhead.
At first glance, the color of the light resembled that of a gigantic bathroom heat lamp. It shone through some translucent material, illuminating the entire space. Golden light spilled over the "walls" and beaches.