The merfolk recoil from Ox’s aggressiveness, and gurgle something in Deep Speech. They remain in the water, unable to crawl onto land without difficulty.

Nylia, this appears to be the final chamber in the cavern.

She looks for anything long enough to reach back across the chasm. Failing that, she props her shield up in front of her (in case it shoots darts out) and pokes the statue with the flat part of her sword.

Nylia pulls a Ring of Water Breathing, a set of Mariner’s Armor, and a Trident of Fish Command out of the water. 

The statue is just that, a statue. It does not appear to be trapped.

Dane looks at Ox, "Peace!  We are already in enough trouble, let's not antagonize them yet.  Let's walk back away from the water's edge and see if they are willing to talk.  We're in their territory.  Do you know Undercommon?

(What kind of Mariners armor?) (Please be splint, please be splint :)

I speak Deep Speech. I should understand what they're saying. 

Nylia dons the mariners armor (I've added Mariners Chain Mail, correct me if it's a different type) and the ring of water breathing. She takes the time to attune to the trident. She then goes back to the waterfall and sees if she can find a good point to tie a long rope to.

Nylia: The mariner’s armor can be whatever you need it to be. You can tie your rope to one of the remaining bridge supports.

Ox: The merfolk, roughly translated, are saying that they have not seen landfolk in a decade, and wonder what brings you here.

I tell them in Deep Speech, "Hello, sorry to scare you. I am Oririeth Xistsrith, from the distant lands of the Forest of Vordorn. If you don't mind me asking, where are we? How do we get out? Can you help us? I can help you."

After tugging on the rope to make sure it holds, Nylia ties all her gear down, ties the rope to her belt and makes sure everything's secure. She slowly lowers herself into the water, testing if she can swim against the current.

Assuming she can, she swims to the waterfall and looks over the edge (holding onto the rope so she can’t go over) to see how steep the fall is.

Dane will look around while Ox is discussing things in a tongue he doesn't understand.  He watches the merfolk for any kind of hostility or other identifiable body language.  He will also try to get his bearings in this cavern while they talk.

Insight 18

Perception 4

I then turn to Dane and assure him, "They're not hostile. I'm just trying to see if they can find my stuff."

"Ah, that makes sense.  If we can get some of the stuff back, that'd be great.  Can you ask them how we can get out of here?"

"I'm working on it," Ox grumbles.

(Sorry guys, I don’t get on D&D Beyond much anymore.)

The merfolk appear to be confused by the questions. “You are here.” One replies in Deep Speech. “To get out you must swim.”

Nylia holds her breath and hits the water. She prepares herself for the plunge from the waterfall, but even so her heart drops into her stomach as she descends the twenty or so feet to the underground lake. On the far shore she can just make out the forms of her two companions, alongside two other figures she doesn’t recognize.

(if the rope is long enough, Nylia would hang the rope off the waterfall to be able to climb down and for everyone to climb back up.)

She activates her ring of water breathing and swims slightly below the surface, keeping an eye looking down for anything approaching her from below in the water.

"Are there any objects on the bottom of the water that you can find? They belong to us. Could you help us swim out by showing the way?"

Dane notices Nylia coming down the waterfall and points her out to Ox.  "She'll probably have to swim through the merfolk in order to get to us."