Troubles in Paradise

Yusko, like in most situations, had the solution to this problem.  That said, he deemed the collateral damage too high so it no longer became the solution.  Instead he would just rely on the most full proof and basic of solutions... smash.

Action: Attack: 15 Damage: 10

Attack: 21 Damage: 5

Bonus Action: Attack: 25 Damage: 10

if any of the attacks hit and sneak damage 9

sorry forgot to say all against S1

A few quick precise lighting fast kicks and punches destroyed the rest of the S1 swarm, eliminating creatures one by one. Small bodies drifted down and got lost among dead humanoids and colourful corals.

S1 is dead (will re-direct Nameless shots into another)

Davis has 2 enemies close by - one on him, one on Trudie.

Davis!

Assume I get an opportunity attack as the swarm moves into Davis's reach...
Attack: 13 Damage: 15

Davis then strikes out 3 more times...
Attack: 16 Damage: 14
Attack: 17 Damage: 16
Bonus action Polearm Attack: 25 Damage: 9

Of course, Davis could (and tried) to hit the swarm coming at him. At first, the creatures were cautious enough to avoid the wide blade, but the closeness of the victim and its resistance to attacks sent them into a blind wild frenzy. The heavy glaive proved to be surprisingly effective against the swarm of small creatures - in a few seconds only half of them were still attacking.

Round 2

Math (always check!):

The Nameless girl sent another couple of bolts into the already thinned swarm around Davis. A few more small bodies decorated the floor of the cave and the swarm paused, surrounded by appetizing but vicious victims.

The other one, though, continued their attempts to get through the defences. One kept digging into Trudie's shell, while another was chewing Hensle's shiny armor:

S3 vs Trudie:  Attack: 24 Damage: 16

S4 vs Hansel: Attack: 26 Damage: 31

Map

 Hansel and Trudie!

concentration check: 14 (close, but no cigar...)

Even the mighty power of Hansel's facial pose wasn't enough to withstand the ferocity of the baby fish with teeth, and his radiant nimbus fades. If he could rip his shirt open he would, but underwater and wearing armor he has to settle for a symbolic imitation. The gesture is sufficient to channel Sune's power and heal the blemishes in his skin caused by the fish (Channel Divinity: Preserve Life to heal back to 33 HP. I would include others, but no-one else is below half their max HP).

Then realising he is just too attractive for his own good, Hansel tries to dissuade the fish from chewing on his body any further. (Sanctuary on self. DC 15 WIS save or attack someone else)

“Can’t be greedy Trudie.” The old tort shrugs to herself when her whip doesn’t force her new friends over, but it got their attention at least.

“What the shell is this?” Trudie would give S3 the stink eye. 

On S3
Reaction: Halo of Spores + Symbiotic Entity
Necrotic Damage: 10 Con ST DC 16

After a very intense face of concentration, bubbles would start coming up from nans shell. They were a doozy tickled when they popped.

Casting Poison Spray Con DC16 or take 16 

S4 Wisdom save: 5 S3 Con saves 21 5

The vicious baby-sahuagin were getting angrier as cleric's blood muddied the water more and more, when the wave reviving energy practically threw them off his body. Looking stunned and a bit lost the swarm shifted at place searching for another pray.

Meanwhile Trudie tried to shake off her attackers with more common approach to the dealing with bloodsucking parasites - poison. It mixed well with water, and at least one of the little devil choked on the poison spores. Many still remained, though.

Math (to check):

Yusko's appearance and Davis's mighty attack changed the mood of the swarm that tried to eat the merc. Quite beaten (closer to the elimination) S2 left the un-bitable Davis and surrounded his potentially more easy-to-eat companion.

S2 vs Yusko:  Attack: 26 Damage: 34

(Mistake!!! Damage is 14)

(An AoO for Trudie!)

Yusko and Davis! Then Nameless begins Round 3.

(Davis can finish S2 or attack S3, no AoO)

Davis uses his reaction for Cloud Rune, moving the damage done to Yusko: instead to the most undamaged swarm.

Yusko: will take no damage due to this.

He then strikes at the swarms attacking his companions. The one closest to him first, then moving to toehrs as needed.

Attack: 29 Damage: 11
Attack: 31 Damage: 14
BA Attack: 25 Damage: 9

Trudie takes a swat with her magic stick.

AoO Attack: 19 Damage: 10 

"Are we punching babies?  I don't know how I feel about punching babies..." Yusko says a he continues to punch what he believes are babies of S2.

Action: Attack: 18 Damage: 10

Attack: 26 Damage: 8

Bonus Action: Attack: 26 Damage: 10

if any of the attacks hit and sneak damage 9

Nameless one will direct her attention to the ones fighting Yusko. She takes aim with her bolts trying to take out as many of the small babies as possible.  "Yes!  Kill the babies!!" she yells.

Attack: 18 Damage: 15

Attack: 7 Damage: 22

+5 sneak attack damage to the first hit. 

S2 cloud rushed to the new victim, allowing Trudie to smash a few "fish" with her stuff,  but before the miniature sahuagin could even reach Yusko, an elaborate rune flashed on Davis's glaive. The captivating light dazzled the creatures and the vicious swarm turned back but not to attack the human or the turtle, instead two clouds mixed together fighting each other.

Davis unleashed the fury of his blade on this double swarm, hitting that soup hard, leaving gaps where the blade passed.

Whether creatures forming swarms were babies or not, but they were killers, quite deserving Yousko's punches. The cloud visibly thinned under his strikes, leaving less than half of one initial swarm. And after the Nameless girl was done with it, just a few baby-sahuagin were still attacking. (S2 is dead)

S3 vs Trudie:  Attack: 10 Damage: 3

But at this moment the relatively unharmed swarm caught the girl's scent, so to speak. Confused by Hansel's magic, they attacked the closest prey:

S4 vs Nameless:  Attack: 19 Damage: 19

Math (to check):

Map

 Hansel and Trudie!  Then everyone else!

(Two swarms left - one on Trudie (almost dead), one on Nameless girl (slightly wounded))

(Hansel's current HP is actually 33. Preserve Life can restore a creature to no more than half of its hit point maximum.)

Finally having been given some breathing room (ironic for being underwater) Hansel is able to see the state of things. Making a few quick adjustments to ensure his clothing is neat and everything is sitting comfortably where it should, he bolsters his friends with some divine healing (3rd level Mass Healing Word - everyone gains 16) and then swims in to attack the fish that just left him (dropping Sanctuary). Attack: 18 Damage: 9

(Now Hansel's HP is 49)

Math (corrected):

 Soft glow covered everyone soothing the pain of injured and lifting the spirit, but Hansel did not stop there and walloped the swarm with his quarterstaff. He definitely hit a few babies with it, while the swarm got itself between two fires now (not that anyhow influenced their aggressiveness).

Trudie - and everyone else!

(Math checks out for me. And the loss of her temp HP also marks the loss of Trudie’s Wildshape Symbiotic Entity bonus)

“Didn’t your parents’ ever teach you to respect your elders.” Trudie would chastise the little critters. Feeling some relief to her old aches and pains, she makes note to thank their handsome cleric.

On S3
Reaction: Halo of Spores Con DC 16 or take 5 necrotic damage

“That’s a paddlin’” The old tort swings her staff against the feisty youngsters.

On S3
Attack: 26 Damage: 9 

Davis strikes out again...
Attack: 25 Damage: 12
Attack: 27 Damage: 10
BA Attack: 16 Damage: 9

Yusko felt the his minor wounds close up and say some of the wounds around the others heal up as well.  He felt a twinge of loss, right there in his coin pouch.  He focused his rage on the viscious babies maybe not babies. 

Action: Attack: 19 Damage: 8

Attack: 19 Damage: 9

Bonus Action: Attack: 18 Damage: 5

if any of the attacks hit and sneak damage 6

Nameless one continues fighting, but the things get their teeth on her and she grimaces in pain.  DC 10 Con Save to maintain Zephyr : 16 

Still able to keep the magic on her, she slips away yet again from the swarm, swimming 35 feet north of them and unleashing two bolts. 

Attack: 23 Damage: 19

Attack: 11 Damage: 20

+6 on a hit if she's able to shoot at one in melee with someone else. 

S3 Con saves 9

No, that was definitely babies, in a sense of freshly hatched poisonous snakes ready to kill and swallow to be counted as babies. Very evil, raging and frenzying baby-sahuagin, looking very much like miniature adults. The swarm of these creatures did not want to lose its big and sweet target, too busy biting and clawing to even notice others picking them up one by one. 

All that was left was the one swarm in the middle of the room, not even trying to pursue the elusive Nameless girl but returning to its original prey - the golden priest. However, after the attack of the girl, there were too few of them left to cause real harm (not that they did not try, of course).

S3 vs Hansel:  Attack: 24 Damage: 7

It was the last thing they did - the group, free from the threat of other attackers, finished them off quite easily and could now enjoy the ... relatively less dangerous waters.

Aftermath:

You could make out the size of the place and the contours of the ships forming it. The corals on the inner wall continue to glow softly and beautifully. The passage between two halves of the broken ships leading up looks almost welcoming in that light, though deathly quiet, as everything around.

Turned out that the chamber is connected to the one you came from in three places. Probably it is more appropriate to talk about one huge cave divided by a few "pillars" in the middle. The broken ships to the north and to the sought are filled with dead bodies (very accurately stored). What the broken halves of big barquentine hold - you would have to find out, should you care to enter them.

Map:

Nameless one swims over to the group, "I'm sorry, I didn't know digging in the coral like that was going to release those swarms.  Is everyone alright?"