Troubles in Paradise

The unexpected attack still did not go unpunished - Trudie saw the small cloud of spores clinging to the creature, and then the biting darkness covered everything.

Yusko with a grace of a dolphin jumped out of the dark cloud, but Davis, brushing off the sting, that he barely even noticed, attacked. Or at least tried to, and was rewarded with the sense, that he definitely hit something - no, someone! The thing he hit was soft enough and definitely moving one, probably even seeing the other attacks coming, because it managed to avoid some of them.

From his vantage point above everyone else now, Yusko could see the whole room: the ink-black clouds - two spheres, pressed together, the half wall with Hansel's weapon and the well of light, and the shark-fin creature behind it. The creature squinted its eyes at the cloud as if able to see through it and bubbled some words to which weather underneath monk's feet answered with a sudden movement. The cloud of darkness, the pieces of corals, the party members  - all were sucked into the sudden whirlpool. The shiny "turtle-rock" ended up in the eye of the little storm and was rapidly rotating there. The uneven walls and protrusions from the ships, forming the room, created additional dangers to be smashed against them.

Everyone but Yusko: DC 13 Strength (Athletics) check or take 13 bludgeoning damage and be caught in the vortex.

The satisfied creature returned behind the wall not straggling at all in the currents.

Math (adjusted):

Map

 Hansel!

Then Nameless and Trudie next round (5).

STR check: 8. Conc check: 12

Hansel is starting to feel a little distressed. It's one thing to withstand a bit of pummeling and risk his good looks. But the additional perils of being underwater - the sense of claustrophobia and the threat of drowning - are getting to him and only made worse by this sudden whirlpool. With desperation Hansel exerts all his energy to swim out of the vortex and into the northern room where these fish-folk are hiding...

Another STR check to escape. I'd like to use Inspiration to roll with advantage: 21

Bonus Action - Spiritual Weapon attack if possible (assuming disadvantage): Attack: 14 Damage: 10

H2 Concentration save: 13

The transparent weapon swam through the still waters of the northern room and track the creature, that jerked but did not let the raging whirlpool to stop.

 Round 5

Nameless and Trudie! (Don't forget the checks)

Athletics vs DC13: 11 
Concentration Con ST: 18 

“Whooop!” As hard as Trudie tried to stay upright, she was swept off her feet into the whirlpool. She’d point with her staff, dictating her moonbeam to H2.

Action: Move moonbeam over H2

Athletics vs DC 13 : 7

Nameless one felt the tug of the water, but she was close enough to the edge that she could escape.  She quickly pulled herself near to the wall she was against and once again followed the wall to escape the darkness. Once free of the darkness, she swam up between the wall of darkness on her left and the wall until she could see something. (35 feet movement to M5)  She glanced around for a target.  (Passive Perception 17, if she sees H2 hiding, she'll attack, otherwise she'll look and listen for her turn : Perception check : 24) 

Attacks if possible :  BA Hunter's Mark on H2. 

Crossbow Attack: 17 Damage: 12

Crossbow Attack: 14 Damage: 6

(Purely for the sake of logic and common sense I'll add fluff, absent in the spell description and interpret "caught in the vortex" as "moving with the flow". Mechanically you remain on your spots)

The attack was too sudden for Trudie to brace herself. Dark water swept her off the coral wall the turtle was clinging to and rushed on this giant carousel, periodically scratching against the corals (down to 50HP by now?). Yet, Trudie's strong will kept the blinding light of the moon going, she even managed to shift it - blindly, but it's not like there were many options. 

Con save: 4, concentration save: 19

Those above could see the light leaving burning marks on the flesh of the monster.

One of the observers was Nameless girl, who managed to pull herself outside the murky waters only to realize the sucking power of the whirlpool. The only option was to go up, where Yusko and Hansel already hanged in relative safety. Once she joined them, she got a perfect view on H2, sending two bolts straight at the target.  It did not kill the monster but the water around the wounds turned dark blue - the creature was losing blood.

At this moment the whirlpool, which had begun to lose its black colour, suddenly darkened again, making it's impossible to see hiding H1.

Trudie and Davis - another poison check. 10 poison damage or half as much damage on a successful DC 14 Constitution saving throw. 

 Davis and Yusko!

 Davis in addition to the poison check has to make DC 13 Strength (Athletics) check or take 13 bludgeoning damage and be caught in the vortex. Also, he can not tell anymore if H1 is around, but knows from all the commotion, that everyone went up to escape the whirlpool, and that H2 is to the north behind the low wall.

Davis athletics: 26

Davis con save: 27

Davis takes a moment to regain his wind... Bonus Action second wind: healing 15 hp

He then attacked again into the darkness...

Attack: 16 Damage: 13
Attack: 19 Damage: 17

Not smart enough to even know she could move out of the whirlwind, Trudie takes it.

Con ST vs poison: 13 
Con ST for concentration: 24 

Reaction Halo of Spores: If applicable 5 necrotic damage to one creature you can see when it moves into a space within 10 feet of you or starts its turn there, if it fails a CON saving throw (DC 16)

(Davis is at full health, Trudie - 45 HP now)

The large figure of Davis stood in the whirling waters like an unmoving stone statue, unconcerned with raging waters around. His blade did not find an enemy, however, he felt scratches of coral pieces against his armour, hinting that a bigger piece can be actually harmful and leaving the whirling water is advisable.

(Davis still have movement left)

Yusko!

As his weapon hit nothing, Davis would figure the enemy moved beyond his reach. He would step out of the whirlpool, picking to move in the direction he had felt the enemy previously.

Yusko, not sure he can do anything about the whirlpool.  He could at least go after the one by the light.  He takes a deep breath and shoots off swimming at blinding speed, speed that may even surprise a fish person.  Then at the end he struck out his arm to try and land a heavy blow and then followed up with a second attack.

Move: 45 feet towards H2

Dash: move 45 feet towards H2

Action: Attack: 9 Damage: 5

24 Damage: 10

will spend a ki point to make that 2nd attack a harming hand attack dealing an addition 12 and H2 will be poisoned until the end of Yusko's next turn.

Concentration saves: 20 14 

Davis, completely losing H1, squeezed into the northern room to get away from the whirlpool.

Yusko found a spot allowing him to attack the creature without diving into the light himself and yet remain protected from the whirlpool by the wall. And so he did - two lighting fast attacks sent bits of barnacles all around and the creature broke, resolving to the simple melee attacks in response. Sharp claws scratched Yusko's armour and the last hit of the clawed fist carried a thunderous power with it.

H2 vs Yusko 1. Attack: 17 Damage: 12
                       2. Attack: 13 Damage: 26

Then H2 darted away from the biting Moonlight (an AoO from Yusko)

 Hansel!

Seeing the creature pop up again, Hansel swims toward it and swings the Ugly Stick at it...

Attack: 14 Damage: 7

If the Spiritual Spanker can move far enough to reach H2, Hansel will do this and attack with it as a bonus action. If it can't get that far, then leave it where it is.
(Attack: 15 Damage: 6)

Concentration saves: 23 9 

Round 6

While the whirlpool continued to rage in the middle of the room (Trudie, another DC 13 Strength (Athletics) check or take 5 bludgeoning damage and stay caught in the vortex) the water suddenly became clear. It looks like H1 is no longer with you or hides amazingly well, no one can spot the creature.

Everyone but Hansel for the moment!

This watery environment confuses the Nameless girl so much....the darkness, then magical darkness, then back to normal darkness, the very water around them swirling and pulling them upwards. Confused, but with a single target still in view, the girl fires her crossbow twice more, hoping to mortally hit the thing.

Attack: 28 Damage: 8 

Attack: 18 Damage: 13 

Davis will move to the closest enemy he can find... He will attack while still at ten feet distance...

Attack: 15 Damage: 15
Attack: 29 Damage: 14
Bonus polearm master Attack: 15 Damage: 8

Guess now’s the right time to try to move out. Trudie swims wherever she can.

Athletics vs DC13: 14 
Concentration Con ST: 17 

If she can see H2, she’d move her beam there.

Surrounded and assaulted from every side, the shark finned creature fell to the attack. The light of the moon now illuminated the cut and smashed body, slowly drowning in now calm water.

The shining turtle-shaped "rock" stopped spinning and assumed its initial position, reminding more than ever a peacefully sleeping turtle that for some reason was shedding magical light. The tentacled creature was nowhere to be seen and only pieces of coral were settling down like giant snowflakes. Right now the room reminded a child toy - some globe filled with liquid and loose pieces, that was shaken up and now calming down. And you were in the middle of it.

Aftermath

Glad that the fight is over, but still a little shaken, Hansel begins healing his companions in case more ugly monsters show up...

Start with Prayer of Healing at 2nd level. Everyone regains 17 HP.

Cure Wounds at 4th level on Yusko for 39 HP.
Cure Wounds at 2nd level on Trudie for 14 HP.
Cure Wounds at 1st level on Nameless for 10 HP.
Cure Wounds at 1st level on Yusko for 11 HP.