(If it was a melee attack, the creature takes 10 cold damage from hitting the Armor of Agathys.)

Damián, jumpy from waiting with no visual of an enemy, reacts in a flash to the invisible attack. As a piece of his frosty armor bursts, he strikes out with his Booming Blade in the direction of the attack.

Attack: 14 Damage: 9

(I’m leaving the roll for Eldritch Blast below so it won’t get mad at me.)

Attack: 6 Damage: 8

(You’d do booming blade. Flavor is fine. I’ll wait for that to see what you do.)

Damian attacks as his armor rests, but everyone watches as he just swings his sword around to no avail.

Lanour, you’re up!

Lanour fires in the general direction of the creatures.

Attack: 11

Second Attack Roll (bc disadvantage): 12

 Does that hit?

Lanour takes aim, and the bolt goes through the space as if nothing was there....

Damian can see a rock just floating upwards, and flinging itself at him! (Ranged)

Attack: 15 Damage: 3

Meanwhile, something hits Tanrick again!

Attack: 17 Damage: 1

Back to you!

As the rock bounces harmlessly off his armor, Damián takes in a deep breath from the air pipe and steadies himself to think.

If they were invisible by magic, and just throwing rocks, then they would appear the moment they attacked. Unless they were using high level magic, but I find that unlikely. Not to mention the rocks are flying through water as if propelled unnaturally. Hence we are dealing with invisible creatures who can also propel objects through water via some magical or... Damián gasps! Telekinetic force!

Damián turns to shout to the other gladiators: “Poltergeists! We must be dealing with poltergeists!”
But, due to the water, all that comes out is, “Pblbtblpbts! Bl Bplst bl bplelbl blp pblbtblpbts!”

Damián reaches into his bag and clutches his Pearl of Power, regaining a spell slot, and swims forward to D9.
He is looking all around him for any disturbance in the water that may betray an invisible humanoid shape that is slightly rippling the water in any way.

Frustrated, tanrick lashes out with his knife at whatever hit him. "I hate invisible creatures! they're ridiculously hard to study!" 

Attack: 25 Damage: 4

(Please use the rules for Underwater Combat in the rules. It’s in the Basic Rules and PHB. Either way...)

Damian heads forward, drops his weapon on the floor (only one free action), and uses magic as he looks around. (If you’d like to find it, you’d use an action to Search)

Tanrick slices and dices, but finds nothing in front of him. It may have moved....

Lanour, you’re up!

(Technically, I only need to have it on my person, that was just flavor. Can I not drop my weapon against my will, please.)

(Sure.)

Lanour stows his crossbow and gets out another short, waiting for the next sign of the creatures.

Lanour draws his sword.
(You may use an Action to Search for these guys, ya know)

Another rock is flung at Damian, this time from behind!
Attack: 13 Damage: 1

Tanrick is attacked again!
Attack: 7 Damage: 1

Back to you!

(At a strong urging from the DM, I will use my Action to Search for these guys.)

Damián, unaffected by the rock, swims forward to C9 to get some air, and while he does so, he is glancing over and searching every inch of the arena- intently looking for any signs that might give away the enemy's numbers or position.

Feeling the rock bounce off me harmlessly I laugh. "nice try fools! now then..." I begin searching for any signs of the creatures

(Search. That Search. Perception Check is needed.)

right, my bad.

18

(Disadvantage)

5

Perception: 2