Four bright, small and powerful darts shoot through the air, hitting with a solid thud as each one finds purchase on the body of this vile creature. It hisses in pain as you all collapse in on it from each angle. It looks panicked and its form begins to ripple and waver- it hisses and spits at you all before making one last ditch effort by leaping at Chubby’s face, attempting to suffocate him.

Dexterity save please Chubbs

"Come at me, bro!":  5 (DEX save)

...glug, glug, glug...

I’m assuming that was evil bad guys turn and now it’s Gregor’s? 

Gregor panics, his rage doesn’t do him much benefit in a situation like this, and he does something impulsive. 

Gregor grabs the nearest box (or if it’s the one Lium’s on, then the one next to it), and throws it with all his barbaric might at Chubby and the evil water. 

(Athletics(?) check: 24)

Crispy ducks deeply as a crate flies over her head, shouting “Chubbs, why do you always get..boxed in..in these situations?!”

Haha, noice. Roll, let’s say a d10+str Gregor for damage.

Flying crate damage roll: 9

Liam quickly sidesteps as the crate his left foot was finding purchase on begins to move.  He then watches in amazement as Gregor bodily lifts the entire thing and hurls it at the creature currently wrapped around Chubby's face.

Is Gregor’s flying crate damage to the water beast or to Chubby, or both?

(Please say both, please say both, please say both.)

"Hey, Chubby." Crispy shouts, "Sometimes you gotta think...outside the box."

Indeed ;) 

A huge, hardwood crate is hurled at Chubbs. The feat of strength is vastly impressive. Chubbs instinctly turns his back and feels the crate burst open on both him and the tracker. Loafs of wrapped up bread spill out into the water. The tracker holds on, clinging to life, determined to take another down with it.

Chubbs, you take 4 bludgeoning damage.

It’s now Chubby’s turn. At the start of which, you take 22 necrotic damage as you go to exclaim something about the fact a crate just got thrown at you- but gulp in some foul tasting liquid instead.

You are currently grappled, so you can take actions as normal or try to break the grapple as your action with a DC13 athletics or acrobatics.

Crispy will pass her turn, unwilling to swing and hurt Chubby when he's almost down - she swiftly turns to Nia and says "Hey, baby doll - how about some fire! Don't worry about Chubby's face, it'll be an improvement!"

"No wait! Push it off your face!!"

Chubby is no stranger to foul tasting liquids (remember that ‘77 drow dark ale he sampled the other day?!), so he’ll use his considerable strength and athletic prowess to attempt to rip the water beast off of his head/body: 17

Nice work. You manage to heave and splatter and rip it off your face- it lands on the ground at your feet with a splat. Its movement has slowed and it looks weak. I think Crispy, you’re up if you still wanted a go?

Here come some more non-magical slashing attacks:

Sneak Attack: Attack: 23 Damage: 12
Off-hand: Attack: 12 Damage: 3

Nice work! Wanna talk us through how you vanquish this foe?

(Oh heck yeah I do! )

Crispy flips her short swords around blade side up as Chubby throws the monstrosity from his lovely face to the ground, and she sees an opening. Placing her swords in a fast X formation from right behind it's head, she slices fast as the creature gets "beheaded" - getting splashed with water or Hells knows what on her face. She turns around, licking her mouth.

"Hey, guys." She says, "I think I got it. Mm! Tasty!"

She high-fives Chubby, purposely missing his hand, because Chubby can't see too well at the moment. "Air high five!"

Good job everyone! That could have gone a lot worse!

As the comotion subsides, it falls silent again. All you hear is the creaking of wood and the soft whistelling of the boiler.

As for what is was, Liam was on the right track to figuring out what it was. After seeing it, you can now identify as a slithering tracker. You know that these are not natural creatures- they are summoned via ritual on a willing target who hungers for revenge and vengeance. The ritual transforms the individual into this creature which still holds its mind and thoughts, but becomes a stealthy assassin ro carry out its bloodlust.

Chubby leans over and proceeds to upchuck a voluminous amount of liquid....goo....

After taking a moment to catch his breath, he lets out a loud BELCH, winks at the rest of the group, and announces "To the nine hells with that abomination!  Gregor, good thinking on tossing that crate.  I like your style!  Very crate-ive."

Chubby then turns around, grabs a loaf of bread from the open crate next to him, and starts eating it while walking back to the ladder.  "I need a beer.  And a bandage."

He heads back to his cot, lays down, and is fast asleep.

"Well, that could have been worse," Liam says slowly as he climbs down from his perch.  He then moves to examine any remnants of the creature, if any of its watery substance remains.

"I knew I'd heard of something like this before. Master Stormheart refers to them as 'Slithering Trackers.' While in nature they are similar to water elementals and weirds, they are in fact monstrosities created by ritual suicide, usually for the purpose of assassinating someone the summoner wants dead."

He pauses in thought for a moment.

"Of course, that leaves us a new question.  Was our dwarf friend here the intended victim of the creature...or was he the summoner?"