400gp

Athos will offer his 100gp if anyone is leaning towards getting a potion of water breathing.  "Maybe at least one of us should have it.  Maybe Zumfir.  I don't know how casting spells might be if you are under water."

When Athos offers, Gimmond starts to visibly sweat a little if any look to him to chip in for a potion. Ultimately, it took him several tries, but eventually he got his mouth to work long enough to say he'd chip in one-no, TWO hundred gold pieces towards the purchase of the potion. "But no more or less!"

Twinkle nods and says that she will chip in the remaining 100 gp to buy a potion. "I also vote that Zumfir should be the one to drink it."

(I did not manage to get a map I liked, but here is a version I am not unhappy with. Just an AI edited version of the main one)

You go to enter through the normal entrance, and the first thing you notice is there is a vertical wall of water that through some magic is not coming through the entrance door. You have no choice but to get wet (That rim around the edge is not real. The entire arena is filled with water). Stepping through, you do see that there are air bubbles coming from the grate in the middle which allows you to breathe if you need to (BA when at the grate to refill your lungs and reset the count for holding breath). The water is 40ft deep, and there is another magical barrier at the top, forcing you to remain underwater for the whole combat.

You have a couple of minutes to explore the new arena, finding this out, as well as choosing where you would like to start.

"Should we start in the middle where the air is?"  Athos says after exploring.

"T'is the most sensible way to handle it; especially if its but beasts will have to deal with." Gimmond replied, more or less in agreement. "Just be mindful not to let them to drag you away from the center as best we can, or will see ourselves falling swiftly one by one, picked off to the last." He said, then took up his new spear and set up defensively near the southwestern edge of the bubbling, grated area.

OOC: Speaking of air bubbles, are there enough to obscure vision and cause some type of penalty? 

With regards to speech, don't forget you are underwater (the arena doors haven't closed in those couple of minutes so you can quickly discuss but once combat starts, you can't talk to each other without starting to suffocate. Unless you have a way to breathe underwater like Zumfir's potion)

The bubbles are not enough to obscure line of sight. It would possibly cause ranged attacks to be more likely to miss as it pushes ammo up, but seeing as that would only be a negative for you guys and being underwater is the challenge I was after, I'll say it has no effect 

Zumfir nods, grateful to the party allow him to take the potion. "My thanks, i will make sure each one of you are in peak condition." He drinks the potion, before scanning around the water, "I agree, but we must also be careful not to be surrounded, we don't know where the enemies will appear . . ."

"But worst case scenario, we can just help each other, breathing with the bubbles. We can win."

You get ready and then the grate opens enough to let some fish swim in before closing. And by fish, I mean one shark and three swarms of piranhas. (and because I wanted to be quick I used images already on Roll20 and therefore the piranhas are toads, and the shark is a snake).

 They act before you get a chance to react, going after the ones they perceive as easiest to eat (least armoured). Two swarms attack Zumfir.

The first get a lot of bites on him (25 to hit for 13 piercing damage). With the smell of blood around Zumfir the second group go into a frenzy (advantage) but still struggle to find any purchase now that Zumfir is expecting it (9 to hit, rolled a 3 and a 4).

The swarm on Twinkle also misses (8) but the shark is able to bite (19 to hit) a chunk (11 piercing) out of Twinkle.

You are all up (and I do follow the same colour coding as the inbuilt roller of red if at least one die is a 1, green if at least 1 is max, and purple if both are met. So don't be too scared about that 13 and 11 being the minimum damage)

Athos moves to Zumfir (between the two swarms) and takes a swing with his battleaxe at one of the swarms for 26 to hit for 11 slashing damage and use some(?) of his air to add a divine smite for an additional 9 radiant damage.

OOC: Can Athos use his reaction on that critical hit to make it a normal hit with his new Guardian Emblem?  And how is divine smite affected by its verbal components underwater?

The reaction to make it a normal hit brings the total damage down to 7 (and damage was wrong originally as I accidentally rolled a normal damage, and so rolled it twice meaning the modifier was also doubled which I only noticed from checking new damage. Actual crit damage would have been 10)

In terms of verbal components, there is no hard and fast rule about it, but based of Jeremy Crawford tweets, you can cast one verbal component spell but it uses up your breath (which under 2024 means you get one point of exhaustion at the end of each turn until you get breath back and they are wiped).

I will assume you attack the Red one (for damage tracking) which you manage to cleave through a good chunk of the swarm, the radiant damage removing more of them (assuming you go through with casting still).

Red is a good target.  I'm fine with that using up Athos' breath.  He'll move to where he can breath when he gets the chance.

With a grimace, Gimmond fought back the urge to let out a battle cry as he swam over to strike at the shark's flank from above it, stabbing and possibly battering it with the butt of his weapon. Then while he had its attention, even if only for a moment, he began banging his spear against his shield. An agonizingly slow process in the water. But the sharp and rhythmic clank of metal against metal served as much as a signal for Zumfir and Twinkles to move it as it was to draw attention away from them.

Mo: 5ft to the left/west of current position, then 10 ft up(so Twinkle has an easier time seeing him without him leaving the shark's threat range)
Ac: One-Handed Vicious Spear vs  Shark(AC ??) - To Hit: 12 For: 10 Piercing Damage plus another 3 from Maneuvering Attack Maneuver for Twinkle.
Ba: Polearm Strike vs Shark (AC ??) - To Hit: 12  For:  12 bludgeoning damage plus another 4 from Maneuvering Attack Maneuver for Zumfir(at disadvantage due to blungeoning under water without swim speed).

Action Surge:
Ac: One-Handed Vicious Spear vs  Shark(AC ??) - To Hit: 18 For: 18 Piercing Damage.
(if 12 from earlier had missed, apply Maneuvering Attack Maneuver from 1st attack to Action Surge attack for an additional 3 damage on top of it and still give Twinkle a chance to move with her reaction before officially taking her turn.)

1st attack to hit will apply Sap to shark, giving it disadvantage on its next attack. 

Quote from ToweringInferno >>

Red is a good target.  I'm fine with that using up Athos' breath.  He'll move to where he can breath when he gets the chance.

 No movement required to breathe based on where you placed yourself. Just takes a bonus action (which smite used this turn)

12 barely hits so both manoeuvring attacks can happen. And in fact, vicious weapon on a polearm master is dangerous. You just about kill it too (2 damage over its hp) 

As the shark snaps for her, Twinkle whirls around in the water, flipping backwards under it, avoiding the bite and then kicking upwards, trying to hit the beast from below. 

Reaction: Deflect Attacks: 12, negating the damage entirely, then spending a focus point to force the shark to make Dex Save (DC 13), or take 12 piercing damage back.

Then the goliath follows this up with a barrage of punches, giving her adversary no time to reposition. Even though this was underwater, she is managing well (unarmed strikes don't suffer the underwater attack penalty).

Action:
Attack 1: 12, damage: 8 bludgeoning
Bonus Action: Flurry of Blows
Attack 2: 19, damage: 10 bludgeoning
Attack 3: 23, damage: 12 bludgeoning

The dex save was a crit. But then the shark dies from Gimmond so the other 3 attacks are on the yellow swarm (and I forgot about resistances so the swarm with Athos only takes 5 slashing. The radiant is not reduced. This is down to the fact it is multiple creatures and therefore magical damage doesn't affect it.

The first of the barrage misses the swarm, the two flurries hit for a total of 11 damage

[Ah I got confused by Gimmonds move, thanks for transferring the attacks, sorry for not taking advantage of the maneuver Gimmond]

Zumfir will wield his mace, hissing from the pain of the bite of the swarms; he quickly infuses it with magic before delivering an attack on the fish:

Attack: 12 Damage: 8 RADIANT damage

Also, casting SHIELF OF FAITH on myself (+2 AC)

(Shield of faith also has verbal so you will have the same suffocation effects as Athos).

The red swarm now only has a couple of fish left. 

The two swarms on Zumfir continue to try attack him. The first one misses thanks to the shield (18 to hit). The second gets another crit for 11 damage (though assuming a reaction to turn it to a normal hit, only does 5 damage)

The one on twinkle gets a 20 to hit, for 9 damage.