There isn't any cover available on the island, no.

Okay, so Tangle is settling in to wait. What are the others doing?

Can Soar extremely slowly start to move forward? Over the course of ten minutes? Would that give any bonus to stealth? Perhaps get within sight of the island to be able to see Tangle?

I don't think moving slowly would in itself give advantage, but you can certainly move very slowly forward. It would be very boring, and you could reach the island about the same time as Tangle re-emerges from the lake, yes. Shall we assume that Bazavur and Jalt also creep slowly in the same direction. It would be quite a sight. You don't need to make a Stealth check, though - no-one sees you.

I don't know how well 2 dragonborn can creep. But following Soar seems the right idea before we all end up split up again.

Luckily there's no-one in this part of the complex at the moment, so two dragonborn can creep sufficiently. You can give me Stealth checks for the heck of it if you want.

Tangle, you see three figures of varying stealth abilities appear from the tunnel to the outside, moving at a comically slow pace.

With a quick glance at the officer's tunnel, Tangle swims swiftly towards the others, pulling herself quietly from the water. She approaches them and whispers, "At least three in the small tunnel where the troops sallied forth last time," she points just to be perfectly clear. "I will return to the island, you should draw them out into this chamber, perhaps say something in the dragon tongue, that they may not be as alarmed as otherwise. Then we kill them." She nods decisively, turns and returns to her spot.

Soar asks Tangle if she saw anyone they recognized from the cult. (Or was there anyone she saw well enough among the battle earlier that she'd be able to replicate?) "I could look like one of them, and speak in draconic, sure. Or maybe... should I look like the prince? I could definitely take on his image, and I have been practicing my voices for a long time, I could try to sound like him. Maybe I could seem confused, or injured, or about to pass out. Then we would know by their reaction if he's here. Immediate distrust and skepticism, he's here. Confusion at his state and condition, he's not. Maybe they come out and try to see to him, and then everyone pounces? Or perhaps have him, or me, be visible to them and draw them out? I could also try to talk to them and maybe learn something, but that didn't work well last time."

She uses mask of many faces to turn into Neronvain, haggard, face drawn, tired, and injured. (We heard him speak draconic, right?)

And she begins practicing his voice, disguised a bit by how she's heard Bazavur and Tangle's normal tones were affected by the pains of resurrection. 

You did hear him speak, and saw him in the battle. You all see Soar become this person. As for nailing the voice, make a Performance check.

Soar performance: 22

If she realizes she's not getting the voice right, she'll say she'd better just grunt and act injured or about to pass out, rather than try to converse. Or just stay silent, and seen. 

Tangle just stares at Soar. “Or we could just wait until they come out on their own.”

The voice isn't going great, and Soar would know that.

Soar nods, and steps back into the shadows of the tunnel. She will wait, still looking like Neronvain. Whether her giving up is a maturing common sense, lessons learned from their ordeal, or hope and joy getting beaten out of her, only Kelemvor would know.

Okay, so the plan is to stay on guard here for... hours? To see if someone comes out?

That's a yes, so...

After about an hour, a long culist, a human man, comes out and takes a look around.

You are, I guess, hidden something like this? Tangle is stealthy but would need to be prone in order to hide.

Remembering how bad things went the last time they were around here, Jalt casts Aid on himself, Bazavur and Soar. 

Yes, that works for placement.

(I thought Tangle had been prone and in the water, with the island between herself and the opening?)

The cultist looks around, and doesn't appear to notice anything amiss. They appear ready to go back into the tunnel.

Tangle watches the others to see if they are doing anything. If the cultist turns their back to return to the tunnel, she will rise quietly from the water to a kneeling position and shoot him.

(That has such a cinematic feel to it!)

Soar silently thanks Jalt for the aid.

She very nearly walks out looking like Neronvain, or staggers forward, or just falls down loudly on the ground as if passing out. She likely would have, before. But she holds herself still. It might be a missed opportunity, but it might also be the thing that gets the cultist to run away, sound the alarm, ruin the surprise, and endanger the group.

So she stays put, watching Tangle and following her lead. She readies an eldritch blast for the minute Tangle fires.