Athos will use his reaction again against the crit. (Really burning through those charges).

He'll get a breath with his bonus action and then attack the red swarm again but miss terribly with a natural one.

Gimmond frowned but wouldn't waste the breath trying to admonish the duo. But, oh boy would he have words for the cleric and monk later! For now, however, he swam his way over to aid Twinkle with her swarm of piranhas had she not dealt the finish blow to scatter them. Otherwise, he goes for the pincer maneuver in striking out at the swarm(orange) attacking Zumfir from the south.

Mo: 5ft NW, then 10ft West from current position (shown currently on last map)
Ac: One-Handed Vicious Spear vs  Piranha Swarm(AC ??) - To Hit: 18 For: 8 Piercing Damage.  
With Savage Attack Feat(which I totally forgot up to now!), taking the 2nd results for a total of 16 Piercing damage instead.
Ba: Take a breath.

It is the round for nat 1s as the fish also get a nat 1 to save against Twinkle for a further 4 (after resistance) damage. They are also injured (<50% health). And as swarms, you notice that once they reach injured, due to being less fish, the damage they can inflict also reduces. Plus a further 5 damage from the bonus action punch. The spear from Gimmond then manages to impale the few remaining fish (8hp left after Twinkle. But also resistance so savage attacker is the reason you killed them).

Zumfir still up (and could quite feasibly bring us to one swarm left)

( wait is it verbal? i'l cancel shielf of faith then XD forgot about it . . .)

Adamantine plate turns the crit into a normal hit, so i'm OK. 

Zumfir will use Healing word on himself (12 healing) and will attack with his mace Attack: 13 Damage: 6

Healing word is obviously verbal, so now you need a breath. And the shield of faith had blocked an attack and therefore he would have taken an extra 5 damage.

He does manage to just about hit the injured swarm, but it takes out the last fish so there is just one uninjured swarm left. Which attacks Zumfir again, missing (16 to hit)

( i'm bad at this LOL.)

Also forgot to say, you are all up 

With only one technical threat remaining, Gimmond really put the pedal to the metal as he raced forth to skewer as many fish as he could, before then smashing into the rest. Swimming in heavy armor was starting to take its toll, as both of his attacks are at times painfully slow, frustrating the dwarf!

Mo: Into melee with final swarm.
Ac: One-Handed Vicious Spear vs  Final Piranha Swarm(AC ??) - To Hit: 12 For: 11 Piercing Damage.  (didn't bother rolling a 2nd time, since doubt a 12 hits in the first place.
But if hit, target is Sapped.
Ba: Polearm Strike vs Final Piranha Swarm(AC ??) - To Hit: 11 For: 12 bludgeoning damage. (Same deal as the first.)

Just narrowly misses 

Twinkle swims over and attempts to disperse the remaining swarm with a couple of punches. She channels the inherent powers of her wraps to catch as many of the fish as possible [switching from bludgeoning to force damage].

Action:
Attack 1: 23, damage: 12 force
Bonus Action [Flurry of Blows]:  
Attack 2: 16, damage: 7 force
Attack 3: 15, damage: 11 force

Athos continues his attack with a 22 to hit for 11 slashing damage and adds another smite as he sees his attacks aren't as effective for 10 radiant damage.  He looks ready to be out of the water.

(coughs in sick)

Zumfir will try and take a breath of air, while attacking the swarm with his weapon normally. 

Atk: 10 DMG: 2

Twinkle takes the whole swarm down by herself. As you start to swim out towards the magically warmed towels waiting for you, the water slowly starts to drain away, carrying the bodies with it, but not strong enough current for you to be affected. 

You find yourselves 400gp richer, and feeling stronger (level up)

Athos will do his usual cheer to the crowd before leaving the arena.  "Well, that was an interesting fight.  I've never fought fish before."

Twinkle grins into the round. "Well done everyone. That was refreshing!"

Still a bit breatheless after so much swimming around, Gimmond can give but a halfhearted wave to the crowd on his way out of the arena. "It was challange, that's for sure. Think I'm gonna need to work on some nonverbal signals with you lot though to better cover our basis in the future." He said, giving the party a wiry grin, then headed out to see if he could procure a ring of swimming for the future. That, and find out what the next match has in store for the group.

Perception: 21. 

Insight: 19 for Athos

The rumours you hear are that you are fighting a creature from the underdark. A drider!

A ring of swimming will set you back 1000gp

"A creature from the underdark." Athos pauses in thought.  "I can't say I've encountered anything like that, but maybe...." Athos works to prepare for the drider.