Oceanus perks up. 

"Sharks," he says, quietly and calmly, but with a sense of urgency, to Gerrard and Qivys. "Multiple."

He nods to the east, from the direction of the river. 

(Make a stealth check if you wish to remain hidden.)

Gerrard:

     Halting his conversation and nodding to Oceanus, Gerrard drifts up to cover as best he can and stays as still as possible.

Stealth:  22

Peri:

Peri watches the antlered bird anxiously until it flies off. "Must be what Dwal saw from the Abbey," he whispers to Kei. "So besides watchin' the sea for the Eels and any betentacled horrors from the deeps, we have to watch the skies for giant killer birds too. Wonderful..."

Kei 

A peculiar sound draws his attention upwards. The swooshing sound comes from a giant deer-bird. Kei hunkers down low as the strange hybrid creature surveys the scene. As the bird flies off Peri's voice breaks the silence and gives Kei a fright. "Shhhh. It can probably hear you" he whispers. "I don't think there's any direction we are safe from. I thought the Styes were a dangerous place, but I'm starting to think it's safer than anywhere around here." He shifts his weight as his cramped position brings a tingle to his left foot as blood flow is cut off. "That lot better hurry up, I've a sense to leave without them".

The mist flows around the enslaved workers, in another easy drift. If there is someone who is out of view of the overseers, Boko pauses and considers trying to un-gas, convince them to drink another, drink the last herself, and try to flee. Seeing how many sahuagin in the room there are, and realizing how long she's been gone, she decides not to actually follow through. She does do the loop around the room, looking down the stairs to see what might be there.

But assuming there isn't an easy target to rescue via potion, out of view, she exits the room and proceeds to the front entrance.

Bokogas circumnavigates the room.  The enslaved laborers are all unarmed and in a weakened state. They cower from their captors; their minds seem scattered. None seem to be in an easy position to rescue via potion.

Looking past the guards, Boko looks down the wide staircase. Cold, black water laps at the second step from the top of the stairs. One of the sahuagin who seems to be in charge wears a pair of gold and silver armbands encircled by intricate designs depicting shark teeth.

(Is Boko returning to the front entrance? Make two more Stealth checks with advantage: one for each guard room you intend to pass through.)

Meanwhile, under the causeway, two shell-armored sharks drift by in the black shallows.

Oceanus stealth: 12
Qivys stealth: 6 

Boko stealth: 17 13

The sharks spin and flash toward Qivys without hesitation. 

Qivys and Gerrard: Roll initiative!

Gerrard and Oceanus are hidden.

Oceanus initiative: 18
Sharks: 2

Boko makes her way back toward the entrance. She successfully passes through the first guard post.

It is at the next guard post - immediately inside the front gates - where she is first detected. She is only 15 feet away from escaping into the outside air.

One of the sahuagin –  weaponless, albeit with the claws and teeth of its kin – glances over at the passing gaseous cloud and makes an arcane gesture.

Boko: Make a DC12 Wisdom save.

Gerrard Initiative:  17

Qivys' initiative: 8

"I hate tha water" Qivys mumbles

Boko WIS save: 24

Peri:

"That lot better hurry up, I've a sense to leave without them".

"I told 'em we'd wait, so we wait!" he snaps at Kei. But he knows he's not really angry with the bard, just worried, so he adds in a more conciliatory tone, "But yeah, I'd be a lot happier watchin' this shore disappear behind us. I hope they're not gettin' into trouble..."

Shark attack!

Initiative:
Oceanus: 18 (hidden; unseen attacker advantage) (Darkvision)
Gerrard: 14 (hidden; unseen attacker advantage) (Disadvantage due to complete darkness underwater.)
Qivys: 13.  (Darkvision)
Two Armored Sharks: 2 (attacking Qivys) (Darkvision)

It is pitch black under the water.

Underwater combat rules:

• When making a melee weapon attack, a creature that doesn’t have a swimming speed (either natural or granted by magic) has disadvantage on the attack roll unless the weapon is a dagger, javelin, shortsword, spear, or trident.

• A ranged weapon attack automatically misses a target beyond the weapon’s normal range. Even against a target within normal range, the attack roll has disadvantage unless the weapon is a crossbow, a net, or a weapon that is thrown like a javelin (including a spear, trident, or dart).

Oceanus gets the jump on the sharks, springing out of the gloom.

Trident attack #1 vs. Shark #1: 24  Hit: 3 piercing damage.
Trident attack #2 vs. Shark #1: 17  Hit: 8 piercing damage.

Gerrard and Qivys may act next!

Only the first of Oceanus' attacks hits, doing minor damage to the powerful beast.

--

Meanwhile, Boko shrugs off a sense of arcane paralysis. Boko: Roll initiative!

Boko initiative: 12

Gerrard:

With Oceanus' warning and sudden movement, Gerrard realizes that the sharks are attacking, but he can see nothing!  Unsheathing his weapons, the glow from his rapier illuminates the oncoming sharks diving at his unarmored ally even as it reveals his hiding place.  In a sudden burst of movement, Gerrard takes Qivys' place as the sharks move in for the kill, placing himself directly in their path but striking a potent defensive stance with the aid of his martial training and magical armor that facilitates his movement underwater.

'What in tha 9 hells are ya doin'?' Qivys exclaims as Gerrard moves the dwarf aside. "I can fight ma own battles!"

With that Qivys casts Bane at 2nd level on the two sharks.

As a bonus action Qivys will touch his holy symbol and say a short prayer, regaining the 2nd level spell slot he just used.