As quiet as a ghost, Boko gets a better look at the west end of the room and its dark contents: several instruments of torture — a wheel, a metal boot with catches and screws, a press, and a rack. By the east wall rests an untidy pile of chains and manacles, leg irons, and bunches of keys.

Boko checks each door for openings big enough for her to slip through. Are there any that are open or that have grates? She will go through those. If none, she will return to the group outside.

Boko examines the five solid cell doors. They are similar to the one she slipped through - with similar-sized cracks along the top that allow the possibility of acro-octo-entry. Only the last door to the far east is unbarred from the outside. All of the doors are 4-inch-thick double doors made of dressed stone, 5 feet wide and 10 feet tall. They open on metal casters that move along grooves cut into the stone floor. The metal bars that lock each door look easily and quickly removable from the outside.

She peers through the crack atop the first door to the east (barred shut). Acrobatics: 6 

...But, alas, she cannot squeeze in far enough through the top of this tight-fitting door to pass through or even get a good look inside. All she can tell is that it is a similar-sized cell to the one she passed through, but she can't look down far enough to see if it's occupied.

Kei 

(If I may be so bold as to retcon a little bit)

As the group waits for the cover of darkness Kei sets about calling forth his familiar. With the hour long ritual complete a final gesture causes the area at his feet to shimmer. Once the refracting light normalises not an owl but a crab stands in front of Kei. Krill, welcome my friend. You will be our eyes underwater,  OK.

(Back to present events) 

Kei, who has been quiet for what seems like days, finally breaks his silence. "I can't take this anymore! What if she's been captured and we're sitting here just waiting to be attacked?" He breaks from behind the ice wall and dares to enter the small chamber. He pauses there and points deep into the tunnel leading away from him. 

@ Boko 

 He waits for a response, a little afraid that one won't come. Krill continues to swim in a broad arc around his master.

Krill, together with Oceanus, Peri, Kei, Gerrard, and Qivys, remain near the ice wall outside the abandoned entrance. The water remains quiet, dark, cold, and thick with sediment.

Boko continues to examine the cell block within. The torturers continue their dark work with their Locathah victim.

Gerrard:

The young noble strives to be patient, but in this underwater realm so unlike the surface world, he struggles.  Drawing strength from his friends, he waits, just a few moments more.

Assuming Boko can hear, she does her best to whisper, if she can, back to Kei. "Long tunnel, stuck slab into back of prison cell. Difficult to pass. Torturers working. Returning now."

She confirms the door to the south isn't one she can sneak through, or see through, and then returns to the cell with the eel, trying to see first if there's a latch or lock she can easily unpin or disengage to make entry easier for humanoids.

The octopus form returns back down the passage to the group, and shifts back to her normal form. She gives everyone an update about what she found.

Boko examines the door to the south. It is closed, but not obviously locked or barred from this side. It's also rather snugly fit, with no crack large enough for an octopus to pass.

The cell door with the eel, like the other cell doors along the north wall, can be unlocked by lifting the bronze bar off its brackets. It would be too heavy for an octopus (STR=4) to lift without help. For an average humanoid, it would be straightforward to unbar.

Boko doesn't try to lift a bar, and returns back to the group per the above...

Gerrard:

What are our chances of alerting others if we go in and ambush the torturers?  The prisoner may have invaluable information…and to be honest, I’d like to save one of our ally’s people if we can do it without alerting whole place,” Gerrard opines rather predictably.

Qivys looks at the nobleman from the side.
"Yer aware this is a scout mission, not a rescue mission an' any going off tha plan ups tha anti about alert them we're here." he asks quietly. 

Gerrard:

     "Yeah," says Gerrard quietly, "Though I'm not sure how much more scouting we can do while they torture the creature in the hall next to what sounds to be the only entry deeper into the fortress."

Boko says “we can take a look at the slab as a group and maybe we can move it without noise… the room seemed self-contained so a rescue may not be noticed.” 

She leads them down the passage and takes another look at the slab without being in octopus form. Does it seem moveable?

The slab is big and heavy, but would probably be moveable by the group if there wasn't a massive corpse pushing up against it. Nevertheless, it is passable by a medium creature in its current configuration, single file. 

Perhaps a bigger issue is the cell door to the torture chamber:  Not only is it barred from the outside, but the huge bloated body of the dead eel is pushing up against it from the inside. It will definitely require a strength check to push inward from the other side, once the bar is removed. And that will be very difficult to do without the torturers noticing.

Peri:

Peri looks between Gerrard and Boko with exasperation. “We’re here to gather information to take back to the bigwigs. Without alertin’ the eels!  Even if we were to somehow manage a rescue without bringin’ the whole damned army down on our heads, dead guards and a mission’ prisoner are gonna get their attention. We’re lucky they think this tunnel is unusable. If they start a real search they’ll find it and there goes the only unguarded way into the fortress!”

”No, it’s too bad the locathah’s got caught, but he’s not our responsibility. It ain’t worth sinkin’ the ship to save one man overboard. We’ll wait til they’re done and then try to find a way through. “

"So you suggest we wait here for them to finish their torture and leave the chamber... and if they do not leave, then we... leave? Or try the other entrance with a garrison inside? Or try the entrance at the top of the causeway?" Boko says back to Peri, quietly. "It is helpful to know of this entrance. The sea elves, and we, could potentially make use of it during an assault, correct, Oceanus? But we also do not know much more than there are dozens of fighters and armored sharks in that larger seaward chamber, and three guards at the end of this forgotten passageway. There is still a lot more fortress that is unknown. If that intelligence is sufficient, then fine. If we need to know more, then there are more risks to face."

Kei 

"We could make them leave." Kei says bluntly. "If their toy was to disappear in front of them I'd say they'd leave. Maybe even without closing the door behind them. Only problem might be that they'd be back in larger numbers. Might be the window we need though." He goes on to explain that he can have Krill approach the captive and cast a spell to make the prisoner invisible.  Meanwhile the group waits safely behind the eel/slab. He waits to see the groups response to his plan.

Peri:

Peri glares back at Boko. "We wait as long as it takes for those guards to clear out. Nothin' says this mission has to be done in an hour, or six, or even a day. I don't like the notion of spendin' any more time than necessary here in the enemy's lands, but it's more important to be thorough than to be fast. Those guards'll go when they're finished -- you didn't see any bunks or a table and chairs or anything, right? So it's probably not someplace they spend a lotta time."

"That bein' said, I like the idea of hurryin' 'em along. But it won't do any good turnin' the prisoner invisible if he's tied to a chair or a rack or somethin'. If you could teleport 'em out maybe, but then the guards'll get all upset and sound the alarm. What about knockin' the prisoner out? No use torturin' someone who's unconscious, right? If you've got a way to knock 'em out the guards'll probably throw 'em back into a cell to recover and leave. Might even think they're dead. Then we can slip in and take a look around, maybe rescue the prisoner on the way out if we have a way to cover the escape route, like pilin' that snake back up against the door. They might think he's escaped and is sneakin' around inside somewhere, but if there's a pile of dead guards in there they'll know he had help."

Ten minutes pass, then twenty. The torturers seem to be taking their time. The locathah on the rack shrieks and cries out in Common as well as Aquan. The sahuagin torturers mock and laugh, speaking only in their own raspy tongue.

(Remind me again the time limits on water breathing, Pass Without Trace, ice walls, and any other time-limited spells?)