Kei 

Kei gives Gerrard a thumbs up to indicate he understood. He, in turn, mimes back that he will speak to the surface to have them raise the bell so it clears the sunken skeletal stern of the Tammeraut. He grabs the speaking tube and yells into it, unsure of how well his voice will carry. "Hello? Raise us up about 20ft. Then 30ft east followed by 10ft north. Slowly. Gerrard will assist in guiding" he speaks in short sentences. 

(Natural d20 roll for his estimates on the distances - 15 )

Kei does his best to maneuver the odd craft from within, and Gerrard from without. In jerks and starts, the bell slowly and uncertainly moves over the yawning hole in the bow of the wreck, as Kei relays instructions to the ship on the surface.

(We'll make this an Intelligence (Acrobatics) check - one of those weird 'Skills with Different Abilities' which is now core 2024 rules. Since Gerrard is helping, either he or Kei may roll again to see if the help indeed helps. It's an Intelligence check plus your Proficiency Bonus if you have acrobatics proficiency.)

Gerrard:

Intelligence-based Acrobatics:  20

The bell grinds against the side of the wreck's interior, loosening years of silt and clouding the immediate water. A few ancient planks snap. But it goes in.

And then chaos breaks loose, as corpses, some with rusted longswords, suddenly emerge from the wreck's crevices. 

A masked face appears in the bell's porthole. It removes its mask, revealing a horrifyingly rotting face. Kei and Kellen must succeed on a DC 13 Wisdom saving throw or be frightened until the end of their next turn. (Gerrard cannot yet see it.)

Roll initiative!
  Three drowned swordsmen: 0
  One unarmed drowned corpse: 21
  One terrifying no-longer-hooded corpse: 20

Gerrard Initiative:  12

Kei 

WIS save 13

Initiative 6

Kellen

Wis save: 19

Init: 9

Initiative:
  Gerrard: 23 (outside bell)
  One unarmed drowned corpse: 21
  One terrifying no-longer-hooded corpse: 20 (Hidden from Gerrard, as it is mostly below the diving bell)
  Kei: 17 (frightened by the hooded corpse) (inside bell)
  Kellen: 9 (inside bell)
  Three drowned swordsmen: 0

Gerrard is up first!

Gerrard:

Releasing the pressure on the diving bell from his free hand, Gerrard draws his dagger and wills it into flame.  Though the fire is suppressed, he draws comfort from the heat radiating off the blade.

The young warrior moves over to engage the closest of the three skeletons grouped just to his north, stabbing and slashing at them with his weapons!  Recalling something about the difficulty of piercing skeletal bones, Gerrard prepares to parry with his glowing rapier, focusing his true efforts to land hits with slashing attacks from the scalding hot dagger!

(Bonus action used to activate the dagger, so no off hand attack this round.  Rolling 1d6 instead of 2d6 since fire is inhibited underwater.)

Scalding Dagger:  Attack: 12 Damage: 9

Scalding Dagger:  Attack: 19 Damage: 13

Scalding Dagger:  Attack: 13 Damage: 9

(Gerrard is flooding the bell?)

With a trio of burning hot punctures, Gerrard badly damages the first, sluggish, dead bladesman.

The much-more-nimble, weaponless corpse scrambles atop the bell and dives on Gerrard.
Unarmed Strike #1 vs Gerrard: 14  Hit: 5 bludgeoning damage, and Gerrard must succeed on a DC 12 CON Save...
Unarmed Strike #2 vs Gerrard: 8  Hit: 10 bludgeoning damage, and Gerrard must succeed on a DC 12 CON Save...
Unarmed Strike #3 vs Gerrard: 24  Hit: 4 bludgeoning damage, and Gerrard must succeed on a DC 12 CON Save...

The hooded figure fires a hand crossbow at the thick glass of the porthole, twice:
Hand Crossbow vs. window: 14. Hit: 5 piercing damage
Hand Crossbow vs. window: 25. Hit: 7 piercing damage
Then it ducks down and attempts to vanish from view. DC15 Stealth check: 11

Kei and Kellen are up next!

The first bolt ricochets off, but the second bolt cracks the window (AC15, 18 HP) deeply. Crit damage: +5
The hooded corpse is still visible to Kei and Kellen, but hidden from Gerrard.

Kei and Kellen are up!

Kellen

Kellen looks over at the frightened Kei as the window near them cracks. Get yourself together, man! Then he looks out the window and disappears from the bell, reappearing 30 feet away and leaving behind nothing but mist. He swirls around in the water and fires off a mote of light at the hooded figure that cracked the window.

Attack: 12 Damage: 12 radiant damage, emits dim light for 10 feet if hit

Gerrard:

(Poorly phrased.  He had contact pressure on the outside of the bell with his free hand to guide it through the hole, and he had to release grip on the bell to draw his dagger)

Parry with the Rapier against that 24 to hit:  7

The shadowy figure easily dodges Kellen's starry wisp.

Kei, now alone in the bell and seriously frightened by the dead face he saw in the window, acts next.

Kei 

Pressed hard against the opposite side of the bell to the porthole, Kei remains fixated on the horror that is the de-hooded figure. He manages a quick glance around the interior, looking for a way to flood the bell from the inside. Whether he sees one or not he returns his gaze to his assailant. He reaches out with his mind and tries to retaliate.

Various action economy related effects plus 14 psychic damagewith an INT17 save for half and no rider effects.

(Kei may flood and open the bell to escape it at any time, but it will take an action to do so. It is also armed with a single harpoon.)

DC17 INT Save: 8

(Kei: burns a Sorc point to quicken the spell to cast it with his BA and then floods the bell with his Action)

Kei unlocks the hatch, flooding the bell. He is ejected into the deep water, only to find himself surrounded by more blue-boiled drowned things.

The three drowned swordsmen surround and attack both Gerrard and Kei:

#1 Drowned Blade: Rusted Longsword Attack #1 vs Kei: 13. Hit: 7 slashing damage, and Kei must succeed on a DC 12 CON save...
#1 Drowned Blade: Rusted Longsword Attack #2 vs Kei: 13. Hit: 11 slashing damage, and Kei must succeed on a DC 12 CON save...

#2 Drowned Blade: Rusted Longsword Attack #1 vs Gerrard: 8. Hit: 8 slashing damage, and Gerrard must succeed on a DC 12 CON save...
#2 Drowned Blade: Rusted Longsword Attack #2 vs Gerrard: 8. Hit: 5 slashing damage, and Gerrard must succeed on a DC 12 CON save...

#3 Drowned Blade: Rusted Longsword Attack #1 vs Gerrard: 9. Hit: 9 slashing damage, and Gerrard must succeed on a DC 12 CON save...
#3 Drowned Blade: Rusted Longsword Attack #2 vs Gerrard: 17. Hit: 11 slashing damage, and Gerrard must succeed on a DC 12 CON save...

Gerrard is up next!

(Attacks against Kei are at disadvantage due to his fancy new cloak.)
Disadvantage rerolls:
 #1 Drowned Blade: Rusted Longsword Attack #1 vs Kei: 21. Hit: 8 slashing damage, and Kei must succeed on a DC 12 CON save...
 #1 Drowned Blade: Rusted Longsword Attack #2 vs Kei: 11. Hit: 9 slashing damage, and Kei must succeed on a DC 12 CON save...

Gerrard:

Having already parried a blow, Gerrard is struck by one of the swordsmen, risking infection by the nasty blue boils on top of his already cursed form!

Constitution Save:  14