((Can he make a Survival check to see if the prints match either sahuagin or kuo-toa feet?))

Make a Nature check to test what you know about foot anatomy and print types between several species.
Make a Survival check to try to follow the tracks.

That'd be a 3 for Nature. You'd think, being so much closer to creatures' feet, Peri would be better at this. I'll leave the tracking to our expert.

Peri is unable to distinguish between these tracks, and tracks a sahuagin or kuo-toa might make. They seem to scrape and smear and drag, and lack any clear toes or claw impressions.

Anyone (or any two) may make a DC17 Survival check if you want to try to follow the bloody trail further, which seem to lead off into the maze of alleyways that surrounds this area.

if Boko, or anyone else, attempts to ID the tracks. Qivys will expend a Bardic Inspiration to her by saying:
"I have faith in ya!" 
((thats a d6 to th roll))

Gerrard:

Not great at tracking himself, Gerrard will defer to Boko, offering her Help as she needs it.

Boko begins to follow the odd tracks south. (Boko Alcor: SURVIVAL: Check: 1d20 (7) + 5 Total: 12.  Advantage roll for Help: 13. Bardic Inspiration from Qivys:_+5)

Qivys' simple pledge of confidence does the trick, helping Boko find the extremely faint traces of the odd tracks as they eventually emerge... directly in front of Dory's warehouse. (The DC was 17!) 

You can't see the suspended boat from this direction, but you do see the front doors of this three-story industrial-looking stone warehouse, dotted with iron-barred windows. Its walls and roof appear to be coated with a bituminous black gruel, an accumulation of the pollution that belches from nearby alchemists’ workshops.

The front of its grim, street-facing facade (perhaps 100 feet wide) bustles with workers who load and unload large, six-wheeled freight wagons from a single pair of double doors. It appears to be a staging area for shipments.

 

"So they're hidin' in there." Qivys says and nods. "Ya want ta send the halflin' an' tha kobold in fer a quick look?"

Gerrard:

”I’m certainly not unobtrusive enough to do so,” responds Gerrard.  “Let me go talk to some of the workers on one side to cause a distraction,” he offers.

Peri:

Peri sizes up the situation before shaking his head. "There's a lotta folks about right now. Harder to sneak in with so many around. These attacks always take place at night, right? Stands to reason the critters we're lookin' for will also be around at night, when there's fewer onlookers. I say we run our other errands, then come back around sundown. We can either sneak in, or we lay an ambush out here. Or both: ambush the killers, then sneak in to see where they're comin' from."

Gerrard:

”Fair enough,” says Gerrard, no specialist in stealthy recon himself,  “Lead the way to our next errand, Master Fenwader.”

Unless you have further business here in front of Dory's warehouse, Peri and Thwack lead you back, uncertainly, through the maze of grimy alleys and streets. Backtracking, they eventually find their way back to Melody's shop.  Under Melody's greasy awning, you see his grimy goods displayed on black wooden racks along the street – tools, mechanical parts, wire, cookware rope, masks, powders, and liquids. Everything is coated in a layer of grime.

Melody, the hunchbacked human merchant covered with warts, emerges from a darkened doorway. "Whatcha need? Whatcha need?" 

Qivys follows along, less than enthused about being led deeper into the dark of this town, keeps a hand on his axe. 

"You want pipes? I got pipes," invites the hunchbacked merchant. "How about a hacksaw? Ointment? I have eleven kinds." Melody crunches on a bag of fried corn. "Masks? Filters? Reinforcement straps? Cable? I got it all..." He catches Peri's eye. "Two sixty," offers the hunchback quietly, again showing just the top of a black blowgun briefly from behind his greasy table.

Eventually, the gnome Grotten appears from a dark doorway. "You wanna talk here? Long night..."

Peri:

Peri gives Melody a long look before answering, "I'll think about it. Lemme hear what our tracker has to say." When Grotten appears he introduces the group. "This is our tracker, Grotten. Let's hear what she has for us. We can do it here, unless you know somewhere more private? Don't wanna go back to our inn, that's for certain."

Gerrard:

Seeing nothing he wishes to purchase, Gerrard nods quietly and follows Peri’s lead in this situation.

Thwack 

As Thwack approaches Melody's store the familiar smell of grease wafts across his reptilian nose. He idles up to Qivys and whispers a word of warning. "Very expensive but the snacks are tasty".

"Okay, let's do it here. You can trust Melody, 'long as it doesn't involve rumcake or somethin'," replies Grotten. Then she lowers her voice.

"Your 'friend' is becoming a known quantity, of sorts. Really nasty fellow, rather eccentric. Party animal. Dangerous type, associating with some rather twisted individuals. Be careful. Hasn't been in town long, but If you intend on 'removing' this guy, just be very, very careful. He's working for and associating with Loquid, the perfumer, of all people, who does a lot of trade with Mr. Dory and is under his protection. So it's possible that your 'friend', who is going under the name of 'Moonbeam', may be under Mr. Dory's protection as well. So tread lightly."

"He's a night owl, hangs around some pretty rough establishments, but doesn't seem to drink, himself. He didn't come into the place I was staking out until after midnight last night, but he was causing drama there until nearly dawn. The kinda guy who's not long for this world, and to be honest I wouldn't be sad to see us lose him. He hangs out with some real lowlifes – cultists, undoubtedly – and spends much of his time in the Low Quarter."

"There's a place called 'The Squat Neck' near Frother’s Lamp in the Low Quarter. Bad news place. If you were to show up, say, an hour after midnight tonight. And not dressed like that" - he points to Gerrard - "or that" - he points to Peri - "and maybe keep your little one here on a short leash, there's a pretty reasonable chance he may turn up again."

Qivys face turns grim.
"He didn't get out of tha vampire's grip." Qivys says in a low voice. "We thought he made out like tha rest of ya but thinkin' on it, there's a good chance he was turned. May not by tha one we killed but one of the others that was there." 
Qivys goes quiet. 

Thwack

The mention of a leash makes Thwacks scales creep. "Snarf! No leash! Doug used to tie up Thwack." He growls deeply at the mention of being restrained. "Me go where Berry or Bernard or Kivers goes. Me do what they wants. No leash".

"Either that, or you can try to shadow Loquid," continues Grotten. "He's supposed to making a delivery, in person, late this afternoon, to Dory's warehouse, here in the Alchemists' Quarter. I doubt Moonbeam will be with him, but he's usually not too far away, these past weeks. Where you find Loquid, sooner or later you'll find Moonbeam. Loquid's been making a lot of deliveries to the warehouse these days. That place has a lot of activity, with people coming and going day and night. But be careful with Loquid. Like I said, he's guaranteed to be under Mr. Dory's protection."

"If you want me to do some more legwork for you, it'll cost you another hundred. You're messing with dangerous folks. Best to let the professionals handle it. I can see what I can learn, if you want me to."