Usul will spend most of the time looking over the edge waiting for sign that the underwater team are back and safe.

Quote from DifferentNoodle >>

Darla sits down, oiling her blade and making sure the gear she's taking is in good working order. (Short rest if possible)

If it turns out that there is time for a short rest, you can take it, but I'll have to let you know later.

And, it turns out that Darla does not have time for a short rest. 

While Usul is looking over the railing of the boat, he sees the shadow of movement under the water. The shadow gets bigger, then turns into two shadows. Then, Nalos and Snurk burst out of the water. 

"We've got three sharkbody abominations coming after us right now," Nalos says, "They're only seconds away. Help us up!"

Usul tosses a rope over to them and runs back to this spot he'd been hiding behind the barrels previously. This time, knowing the threat is coming, as he ducks down he casts an illusion of an additional barrel that he disappears into.

Athletics for Nalos (Snurk is good to go with his natural climbing ability) DC12:

• 14

Okay. Both Snurk and Nalos make it onto the ship. As they get onto the deck, you see the sharkbody abominations breach the surface. You're not 100% sure, but it looks like they're sneering. Then, they duck back under the water.

Snurk jabbers loudly in a bunch of squeaking froggy sounds of Grung, before turning back to the others. 
“there’s a sea witch. Center of the old island, chanting away with several guards. We shall kill them.”

"A sea witch? Did it look like she had a rod with 'er?" Darla asks, blade drawn. "Ah s'pose either way, she sounds like somethin' what needs stopping, aye?" She pulls out her potion, looking around. "We've a target, th' lot o' ye ready tae get wet?" She asks, waggling her brow with a smirk.

Usul peers out of his illusion, and seeing that the ship seems not under attack, he listens to what the recon team is sharing. Understanding that it's about time to get wet, he dashes to his cabin to grab his bubble ((it's a bit big to just carry around I think)), as well as reaches into his pocket to pull Dib out. Setting him on his bunk in a small pile of blankets he tells the mouse to wait here, gives him a bit of cheese and bread he'd be saving, and goes back out to join the group. He then says "I'm ready."

Quote from DifferentNoodle >>

"A sea witch? Did it look like she had a rod with 'er?" Darla asks, blade drawn. "Ah s'pose either way, she sounds like somethin' what needs stopping, aye?" She pulls out her potion, looking around. "We've a target, th' lot o' ye ready tae get wet?" She asks, waggling her brow with a smirk.

 “Right-o. Looked like she had some worshippers, too. She’s getting up to something with her chanting.”

((If y'all are ready to hop into the water, I'll have to post that a bit later when I'm at home. Right now I'm on mobile.))

Not long after you all descend into the water, Nalos and Snurk see the familiar sight of what remains of the sunken island. The rest of you see that it is wedged precariously in the undersea trench. The skeletal frames of its houses still stand, and the gruesome remains of villagers are strewn about.

Near the center, five sahuagin kneel within the ruins about 20' away from a sahuagin wearing a magnificent headdress. You can assume that this is Selachai. She is chanting something in an unknown language and holds aloft a gleaming scepter tipped with a glowing amber eye.

The three sharkbodyabominations are positioned even further away, looking in your direction, clearly now expecting that you would follow them.

Initiative time!

• Darla (AC14): 7
• Snurk (AC15): 8
• Stanley (AC11): 21
• Usul (AC14): 21
• Nalos (AC15): 19
• Sharkbody Abominations (AC12): 7

Notes about adventuring underwater: The deep, turbulent water of the trench makes movement difficult for creatures not used to it. A creature without a swimming speed must spend 2 feet of movement for every 1 foot it moves. This basically means everyone except Nalos. Snurk doesn't actually have a swimming speed like I expected him to.

Initiative Order:

1. Usul
2. Darla
3. Sharkbody Abominations
4. Snurk
5. Stanley
6. Nalos

((I rolled initiative for the others elsewhere and I'll add them in as they become pertinent. Right now, it appears as though only the abominations are engaging.))

Map - Round 1, Turn 1 - Beginning of Usul's Turn

Usul will swim straight west as far as he can to put others between himself and the creatures. He then readies his crossbow to fire as soon as any get into a confrontation with any of his companions.

Usul, being underwater and not having a natural swim speed can only move 10' West (25'/2 rounded down). Since Nalos is in that spot, I placed him one square south of Nalos. Your action is readied. It is now Darla's turn!

Darla, floating in the water, knowing that they're aware of them already, makes eye contact with the nearest Sharkbody. She straightens out the best she can while floating, sword in one hand, and then slowly, with a big shit eating grin, raises a hand and flips them the bird. Vicious Mockery (DC12 WIS 2 )

Then, still flipping them off, looks at Nalos and winks.

(To Nalos, Bardic Inspiration: As a bonus action, a creature (other than yourself) within 60 ft. that can hear you gains an inspiration die (1d6). For 10 minutes, the creature can add it to one ability check, attack roll, or saving throw. This can be added after seeing the roll, but before knowing the outcome.)

Sharkbody Abomination 2 WIS save: 17

The aberration looks at Darla confused, likely wondering what that middle finger was supposed to do, but then he gets this quick, sharp pain right behind his eye for a moment and then it vanishes. He takes the 1 point of damage.

The sharkbody abominations are next.

Sharkbody Abomination 1 will go their full movement speed towards Darla, but won't quite make it.

Sharkbody Abomination 2 will move towards Snurk and attack:

• Attack: 19 Damage: 8 
• The creature "lunges" awkwardly and the attack misses Snurk.

Sharkbody Abomination 3 will go their full movement speed towards Nalos, but won't make it all the way.

It is now Snurk's turn.