(Sorry ignore those last two roles; the second drowned blade, #3, is incapacitated.)

The shimmering, translucent fist cuts through the water unimpeded, smashing the half-hidden assassin back into its hiding spot in the old wreck. Timbers crack, plumes of silt cloud the water.

The lone unhypnotized dead swordsman attacks Gerrard, undeterred:
Drowned Blade #1 Rusted Longsword attack #1 vs Gerrard: 18.  Hit: 6 slashing damage, and Gerrard must succeed on a DC 12 CON Save...
Drowned Blade #1 Rusted Longsword attack #2 vs Gerrard: 11.  Hit: 10 slashing damage, and Gerrard must succeed on a DC 12 CON Save...

Gerrard may act next!

Gerrard:

With only one active foe at the moment, Gerrard manages to just barely deflect the swordsman’s attack.  Nodding his thanks for the assist with the swirling patterns, Gerrard launches into his familiar attack pattern at the active foe!

Scalding Dagger:  Attack: 10 Damage: 12

Scalding Dagger:  Attack: 10 Damage: 11

Scalding Dagger:  Attack: 29 Damage: 14

Glowing Rapier:  Attack: 23 Damage: 12

It takes all four blows, but Gerrard manages to destroy the sluggish-but-tough Drowned Blade with the last hit of his trusty rapier.

Or does he? The boil-covered corpse tries to reassemble itself for a moment... DC17 CON Save: 6 to reanimate...

...but fails. 

Combat is paused, as the remaining three undead defenders of this wreck, each damaged from this combat, stare idly at the swirling grey patterns...

Kei and Kellen are technically next in initiative order. (There is no advantage upon hitting an Incapacitated foe, by my reading of the rules, but it will wake them from their stupor. There is less than a minute remaining in Kei's spell.)

Kellen

Kellen's large, spectral hand floats over as his real hand moves, now outstretched. He wraps the hand around the closest incapacitated undead thingie and squeezes:

Strength check for grapple: 17

Crush damage: 11 bludgeoning

Kei 

Satisfied that the spell has slowed the battle somewhat, Kei follows suit and attacks the same drowned undead as Kellens giant spectral hand. He moves the tentacle (if needed) to within attacking range and lashes out.

A - Eldritch Blast at the undead

BA - Tentacle move/attack against same creature

Rolls in log

The dead, hypnotized swordsman (#1) instinctively tries to wriggle out of the massive, shimmering hand. DC17 Dex Save: 17 

Meanwhile, Kei pulverizes it with a trio of crackling beams. It tries to reassemble itself... DC19 CON Save: 9

The dead Swordsman (#3 not #1, sorry) does not reanimate.

Two remain - both charmed - the drowned ascetic near Gerrard and the drowned assassin, half-hidden in the wreck.

Kei may redirect that tentacle attack against one of the others, and Gerrard may act as well. 

Gerrard:

Following in after the tentacle attack, Gerrard attacks the undead that has been released from the hypnotic spell.  With his familiar attack routine, he strikes!

Scalding Dagger:  Attack: 16 Damage: 10

Scalding Dagger:  Attack: 18 Damage: 9

Scalding Dagger:  Attack: 13 Damage: 10

Glowing Rapier:  Attack: 19 Damage: 9

Gerrard makes quick work of the unarmed undead in front of him. ...But it attempts to reanimate: DC14 CON Save: 4

One charmed foe remains. 

Kei and Kellen may act against it.

The underwater battle is nearly won. A single, half-hidden undead assassin remains, hypnotized by Kei's grey vortex. Kellen's shimmering giant hand hovers nearby.

Kei and Kellen may act next!

Kei 

Kei swims quickly towards Kellen's position. As soon as he spots the final dazed undead he stops. Contemplating unleashing his wrath on the creature, he takes a moment to think through the result. He decides to wait until Gerrard is within range just in case the assassin wakes and has time to escape.

M - swims to within eyesight of the enemy

A - Readies Eldritch Blast for after Gerrard engages the assassin

BA - moves the tentacle towards the enemy

Kei approaches and readies his attack, watching Gerrard for his cue to strike.

Kellen and Gerrard may act next!

Gerrard:

Swimming towards the hypnotized assassin, Gerrard launches into another flurry of attacks.

Scalding Dagger:  Attack: 22 Damage: 14

Scalding Dagger:  Attack: 14 Damage: 9

Scalding Dagger:  Attack: 19 Damage: 9

Glowing Rapier:  Attack: 20 Damage: 12

Kei 

With his friend now fully engaged with the enemy, Kei unloads the built up shadowy energy.

Fumbling with his blades in the swirl of disturbed silt, Gerrard nevertheless delivers a pair of strikes against his undead foe, snapping it out of its stupor.

But before it can act, Kei, ready for it, delivers a pair of crackling blasts and finishes it off. It attempts to reassemble... DC18 CON Save: 19

...And succeeds. But then Kei delivers his third beam, again blasting it to pieces. And again, it tries to reassemble. DC17 CON Save: 16

...and nearly succeeds, but ultimately falls inert. Combat is over; everyone may act.

You remain outside the wreck, in the dead, bitter cold waters some 300 feet below the Gloom's Wake. The flooded diving bell is partly wedged in the hole where the Tammeraut's bow once was. The sandy bed around the wreckage is scattered with partially buried bones and debris, some of which looks fresh. The wreck itself remains unexplored. 

Gerrard:

Taking a deep breath to recenter himself (Second Wind), Gerrard goes over to his allies and offers, “I can tug on the line to send the message ‘hold here’ for the diving bell.  Now that we are all out it likely makes no sense trying to lower it further, and we should let the captain know we survived the initial battle.”

Second Wind:  15

Glancing at the bodies, he considers searching them for clues or useful items…

Perception:  14