Peri:

"If it's as murky as Boko says, we may all need her to lead us. We can hold onto a rope so we don't get scattered. There's a chance my gift might work, but I've never tried it out in cloudy water. And once we're in, are we going to look for the route to the gates? Or do you have other ideas? I've never snuck in somewhere just to look around."

“We are not to be seen, so why not wait until dark and then slip into the water without guards or lookouts spotting us. Maybe most of them will be asleep? Then I can lead us to the underwater forgotten entrance. Holding onto a rope in a line would be smart. I can swim well enough to tug the rest of you through the water, above the ocean floor. I can see underwater in the dark well enough with my goggles, or if I’m a giant octopus or something else that doesn’t mind the dark. Check out what’s coming as we go and enter. Or use your sword to give a little light, especially if no one can see. Then we creep around and either get a sense of the fortress without being noticed, or take out a couple guards we encounter… or we stumble into the army and fight out way out and hope we can get to the ship? That last possibility is the one we do not have a lot of options for. Or… I just lead you to the door as backup and I go scout around as a small octopus and sound the alarm if something goes wrong and I need support.” Boko says.

She will renew her water breathing ritual as they wait, not wanting to risk anyone getting even a bit close to running out of air.

Qivys dons his Helm of Underwater Action and says:

"Alright, I'm ready!" 
He keeps his axe and shield stowed on his body till needed. 

Peri:

"I can wait for dark. No hurry to go stickin' my head in a shark's mouth. Got any druid magic to keep the bugs off though?" He opens another ration and begins eating the unappetizing food for something to distract him from the sense of foreboding.

Gerrard:

Redistributing his equipment for underwater travel, Gerrard pulls out his rope from the bag of holding before closing the bag up tight.  He starts knotting the rope so they can hold on easily as they are guided through the dark waters below.

”I do not know the sleeping habits of the sea devils, but a nighttime scouting trip may indeed be better.  I will defer to your judgement,” says Gerrard, “though once we are inside, I may draw my glowing rapier if the light wont give us away.”

As the gloom deepens, you begin your approach. Slipping into the water behind cover of the head of stunted trees, the water is black, ice-cold, silt-filled, and foul-tasting. There are no signs of patrols or spies. Dropping deeper below the surface, the water becomes even murkier.

(To the smaller, submerged entrance? What form is Boko in? Any other preparations you want to make before entering? What's your "Marching order"? The entrance in and out of the abandoned room is about ten feet wide, so you may swim two abreast.) 

Gerrard:

Holding his section of the knotted rope, Gerrard offers to be first in the tow line behind Boko.  “Good fortune to us all,” he says briefly just before his head submerges beneath the water.

Before the group leaves the trees, Boko casts pass without trace on the whole party.

Boko stealth: 21

She enters the water as herself, trying to ease the transition for the others. "We should be able to talk under here. Can you hear me?" she asks once they are under. She tries to determine if this foul taste is natural or not. 

Nature/arcana: 15

As she swims, almost without effort, just above the seafloor, she uses shape water to change the opacity of the water immediately around them to crystal clear. "Gerrard, try your sword. Perhaps we will see something on the seabed that was obscured by the silt." She can only do an intermittent pathway, 5 feet wide, that quickly mixes with the rest of the water as the waves surge, but she hopes that helps them still be protected from view by the surrounding murk. This is more to pass the time and satisfy curiosity than anything else. She stops when she starts to get anywhere near the causeway, let alone the door, and asks Gerrard to put his sword away. She relies on her goggles to guide them on the path she's almost memorized from her previous scouting, and her hours of staring at the shore from their hiding place.

Assuming she gets there without a problem, she casts shape water again in the silty, dark water, creating a ten-foot-long wall of ice about ten feet from the wall of the fortress, attempting to give the party a hiding place. should any passing patrol or predator happen by. They can crouch behind it and hopefully the ice blocks any thermal signature for darkvision.

She pauses for Peri's devil's sight to detect anything out of the ordinary, and any other preparations the group wants to do.

Then she picks up a small stone from the ocean floor, turns to the group, and bubble-whispers. "Stay ready just outside this door. I can go look first. I'll be harder to spot in this form. If you hear this rock banging against a wall, come swimming. Running. If you see an ink cloud, get ready to bolt. But I should be back to tell you what I see in there, and if the coast is clear for us to enter."

She pauses again for any further contributions from the group. Assuming none, she then shifts into normal octopus form, shrinking down to a smaller creature that picks up the small stone and then changes color to match the stone of the fortress. She pauses, listens, and assuming she hears nothing odd, she slowly moves in.

Perception: 26
Octostealth: 19

Gerrard:

Following instructions, Gerrard gets used to swimming and talking while breathing water.  He does chance a little light in the small clear spaces Boko creates, trusting in the silty water surrounding them to cover its soft emanations from further away.

Perception when lit:  17

The party carefully advances, finding neither guards nor resistance. Peri picks up nothing with his arcane sight; nor does Gerrard find anything of interest on the seabed by the light of his sword.

Scouting ahead, Bokotupus finds herself in a small, abandoned chamber filled with natural litter - sediment, seaweed. On the opposite side, another rough-hewn passage leads out of the room.

Boko the not-so-giant octopus swims through the room like a ghost, then heads down the passage, sticking to the east side of the passage, flattening herself against the wall as needed to camouflage herself.

Let me know as you need more stealth or perception rolls...

Boko makes her way south down a crude, abandoned corridor. It ends with a broken stone slab, dislodged to leave a small gap. Beyond it is the massive rotting corpse of a large sea serpent of some kind, pressed up against both slab and gap. Boko can spy just a little more past the corpse of the small cell beyond.

Then she hears a shriek of pain, and the raucous hissing of sadistic pleasure. Something is being tortured not far ahead.

Peri:

Head on a swivel, sure they'll be spotted, Peri follows Boko's lead. Arriving at the small entrance he hunkers down behind the Ice wall -- barely any colder than the water they swim through, at least it feels that way -- and listens as the normally quiet and somewhat passive Boko takes charge. As she transforms into an octopus and disappears into the tunnel he says quietly to Gerrard, "I don't know what god's eye you've caught, nobleman, but finding that one was a divine gift. I don't know how we'd've pulled this off on our own." Then he listens closely for the rock signal.

Gerrard:

Still getting used to speaking underwater, Gerrard replies quietly, “Truly spoken.  And to get the navy of Seaton, the merfolk, and the sea elves to all help the Lizardfolk reclaim their home—that is a wonder as well.”

Nodding to Oceanus, he adds in Elven, “Many thanks to you for convincing your people to help.  Can you sense anything about these waters that we may have missed?”

(Assuming there are no other portals, holes, or entrances in this passageway...)

Boko looks for a place to slip through the gap in the slab door (octopuses can fit through a hole the size of a quarter), trying to stay out of sight of whatever's torturing, and whatever's being tortured in the next room.

Boko easily slides through the gap between the slab and the wall, and between the massive corpse and the ceiling.

You are in a black, decay-filled cell. A massive eel of some sort was tightly confined here before it died - it fills most of the available space. The water is foul here. A double door leads south. Through the door, you can hear the screaming of a victim, the laughs of multiple tormentors, and the clank of mechanical torture equipment. There is a tiny gap along the top of the door, but not enough to see through (and squeezing through it as a little octopus would require an Acrobatics check.)

Boko tries to fit over the door.

Acrobatics: 6

(What's Bokotopus' vision here, presuming somewhat cleaner water – 30' darkvision, right?)

Bokotopus cleanly slips through the narrow gap over the door (which, she notices, is barred from the outside.)

This long room has at least several doors along the north wall, secured with metal bars like the one she just slipped out of. Another, slightly larger door leads south.

Daring to venture a little west (roll another Stealth check!), she dimly spies three sahuagin with their backs to her, attending to their task, stretching a fish-like humanoid on a rack. As you watch, a wheel is turned, the victim emits another shriek, and the torturers hiss with pleasure. The three beefy sahuagin are each armed with spears.

Qivys treads water for a bit. 
"She off on a site seein' tour?" he ponders outloud.

Boko moves to the west for a slightly closer look but then east to get a sense of the rest of the room. She’s not doing a deep look, much more of a larger, shallower scouting mission.

Stealth: 20