In a moment of clarity, Gerrard locates a potion, feeds it to his dying friend, then leaps onto the neck of the horrid worm.

Kei tries to puncture the interior of the worm's gut from within with his huge giant claws. Alas, he does not succeed in piercing it on his first attempt. (AC>14).
"Join us!" cry the invisible disembodied voices from within the massive worm.

Kellen wakes up prone on the listing deck, in a state of continued confusion. Kellen d10: 2

Kellen is lost in a world of confused hallucination. He cannot move or act this turn. (But at least he is alive and conscious.) He may make another DC16 Wisdom Save versus the confusion, as may Gerrard.

The creature redirects its attention from the warship to the noble clinging to its neck. Its tentacles grasp at Gerrard:
Tentacles Attack vs. Gerrard: 30.  Hit: 27 bludgeoning damage plus 15 psychic damage, and Gerrard must succeed on a DC 18 Strength saving throw or be swallowed like Kei.

Kei takes another 21 acid damage from with its black gut.

The warship sails past, no one at the helm, much of its crew dead. A few uninjured crewmen appear from below decks, but are in a state of near-panic as they try to survive this combat.

The tentacles tear at Gerrard with an unearthly fury. (Crit damage: +16 bludgeoning damage and +17 psychic damage.)
If Gerrard is still up, he must make a DC 18 Strength saving throw or be swallowed. He may also make that DC16 Wisdom Save to end the confusion. (No reactions or bonus actions allowed yet.)

Kei, blinded andrestrained, may act.

Kellen should roll that DC16 Wisdom Save versus the confusion. If he succeeds, he can act normally. If he fails, he must roll a d10 before proceeding.

Gerrard:

Heavily damaged, Gerrard tries to resist being swallowed by the massive creature!

Strength Save:  13

Gerrard:

Wisdom Save versus Confusion:  3

Kei, slowly dissolving even in crab form, senses Gerrard's arrival nearby in the belly of this gargantuan aberration. (They are separated by about ten feet of hard, slimy, black flesh.)

Gerrard may roll a d10. (On a 7-8, he makes a melee attack on the worm. On a 9-10, he may act as he wishes. No bonus actions or reactions.)

Kei, blinded and restrained, may act.

Kellen, now conscious, should roll the DC16 Wisdom Save versus confusion. If he succeeds, he can act normally. If he fails, he must roll a d10 before proceeding.

Gerrard’s Confusion:  1

(Edited to force the confused die roller to work)

Kei 

Kei-crab does all it can - snip at the muscles holding it in place. 

A - claw attack 12 for 34 damage

Gerrard is compelled to try to swim forward, exploring his undersea hallucination. He feels his skin shedding. He tries to swim down to his home, deep under the sea. (He may make another DC16 WIS Save vs. Confusion)

Kei-crab claws desperately at the wall of flesh, but still cannot puncture it.

Kellen should also roll the DC16 Wisdom Save versus confusion. If he succeeds, he can act normally. If he fails, he must roll a d10 before proceeding (acting normally on a 9-10, sans bonus action or reaction.)

Gerrard’s Wisdom Save:  10

Kellen wis save: 12

-_- d10: 3

Kei-crab and Gerrard each take 21 acid damage. 

The worm now turns its attention to Kellen, who still lies prone on the deck of the listing ship, deep in a dream world.

Its eyeless head approaches him curiously, almost gingerly, before striking. Tentacles Attack vs. Kellen: 27. Hit: 17 bludgeoning damage plus 12 psychic damage, and Kellen must succeed on a DC 18 Strength saving throw or be swallowed by it like the others.

The warship continues to run, now out of immediate reach by the worm. Some survivor on its deck manages to fire off a ballista:
Bolt Attack vs. Sea Monster: 17. Hit: 6 piercing damage.

Kei may act!
Gerrard and Kellen may roll a d10 before proceeding. 

Gerrard:

The young noble struggles against the imaginary current until a wave of caustic ‘water’ knocks him unconscious.  He drifts in an imaginary sea, though the amulet he wears stabilizes him (at zero hit points).

d10:  3

Wisdom Save:  9

Kei 

Crabs aren't the deepest thinkers, but they do know when they're cornered. Kei-crab does the most natural thing - snips again at the worms insides.

A - claw attack 19 for 27

I believe Kellen is once again at 0 hp, and now swallowed by the worm. (Correct me if I'm wrong!) He may roll his first new death save.

Gerrard, too, is unconscious, if stable, even as the acid continues to eat at his diseased skin. He dreams of the deep, of dark undersea cities of unimaginable magnitude and alien-yet-somehow-familiar complexity. It calls to him. It feels good to go home.

Kei-crab desperately tears at the acid-oozing black flesh, finally managing to tear at it somewhat. It twitches and jerks, bucking the massive crab form, sandwiched in walls of flesh. Clearly the worm felt that.

Everyone takes 20 acid damage (and an extra failed death save for Kellen.)

None of you has any sense of what the worm is doing, nor what is happening outside.

Kei may act again.

Kei 

Claw attack 25 for 26 damage

(Nope, unfortunately that tracks!)

Death save: 11

Kei-crab tears another huge gash in the wall of flesh. Again it bucks violently.

The acid continues to ooze. Kei-crab takes another 22 acid damage.  Kellen fails an additional death save. Gerrard is at peace within his mind, even as he feels his skin vaguely tingling. He swims freely, his sleek tentacles perfectly attuned to the pressure of the black deeps. He is slowly approaching the twinkling lights in the immeasurable distance.

Kei may act again. Kellen may make another death save (potentially his last...?)