I'll wait for input from the others, but it seems there is 1 vote for a search of the two pools.

LANDMOP agrees to searching the immediate area, but they are tapped for resources and there undead ally would rapidly disappear in the next few moments

Erix votes for exploring; they still have the Elk and the snake.

((btw DM; how do you want to handle the till dawn part of the bag of tricks here in the underdark?))

I think with the Bag of Tricks it should probably last until your next Long Rest.

So which pool will you search first? The one the Troll was retreating to at 9, or the one a little further away at 10.

I suppose the only thing you have to go off is that the Troll thought it could somehow get an edge over you all by running to the pool at 9.

LANDMOP got a 9 for investigation the pool at area 9

(Bonus for matching the map area number?)

Giant Crayfish Stealth: 7

(Haha, unfortunately 5e doesn't reward matches.)

LANDMOP begins searching the pool for things of interest, unfortunately the waters are dark and murky. Additionally, the warforged's refelction in the pool is disrupted by two sets of large claws as two Giant Crayfish emerge from the water, intent on clipping off pieces of you. Initiative:

Group 1: Erix, Mark, and Zoveldra

Group 2: Giant Crayfish

Group 3: Elero, LANDMOP

Erix, Mark, and Zoveldra are not surprised and can act as the Giant Crayfish emerge.

The elk and Snake try to attack the emerging crayfish:

The Elk uses charge:
To hit 19 for 13 dmg and extra charge damage: 5 so total 18dmg and a DC14 str save or be knocked prone.

The Snake than moves in(Advantage if prone): 

8,9 so both miss probably for 15 dmg after which it tries to constrict

DC14 strength save or take 12dmg and be grappled

Erix stays back and throws out another thorn whip at the same crayfish without pull:

to hir 11 for 3dmg

Zoveldra moves to attack the nearest crayfish without skipping a beat.

Action: Unarmed Strike to hit 13 even if advantage

Extra Attack: Unarmed Strike to hit another 13 or 12 without advantage -.-

Bonus Action: flurry of Blows Topple. First attack to hit 24, damage 11 force. Second attack to hit 18 or 25 if advantage, damage 6 force. DC 14 dex or prone (if not prone already).

Crayfish 1 STR Save: 16

Crayfish 2 STR Save: 5

Crafish 1 DEX Save: 12

Erix' Elk hits Crayfish 1, but the rest of his crew miss. Zoveldra's attacks topple the Crayfish and it immediately starts sinking back into the water. Mark fires a shot at Crayfish 2 but it flies wide.

Crayfish 1 swims back up and attacks Zoveldra and the Elk, while the other attacks LANDMOP and the snake. Trying to Grapple and pull whoever is hit underwater.:

Claw against Elk Attack: 15 Damage: 11 Bludgeoning and target is Grappled.

Claw against Zoveldra Attack: 23 Damage: 12 Bludgeoning and target is Grappled.

Claw against LANDMOP Attack: 6 Damage: 10 Bludgeoning and target is Grappled.

Claw against Snake Attack: 13 Damage: 11 Bludgeoning and target is Grappled.

OK, so I think that probably hit the Elk and the Snake. Both Crayfish would try and drag grappled creatures underwater, so I think LANDMOP and Zoveldra could take Opportunity Attacks against them if they want.

Otherwise, you all are up. The Elk and the Snake were just dragged underwater by the two Giant Grayfish.

Elero is definitely missing the city streets above.  He takes a deep breath and aims a shot at Crayfish 1.

Light crossbow attack: 10 to hit and 12 damage.

The Protector Cannon pulses out 10 temp HP for the party members not being dragged under.

LANDMOP will launch a Ray of Sickness at Crawfish 1

22 to hit 8 poison damage

The Elk and snake try to hit their attackers. (Snake has a swimspeed, Elk doesn't if it matters)

The elk: crits for 22dmg
Snake: To hit 17 for 12dmg and constrict DC14 strength save or be grappled and take 11 dmg

Erix tries to pull the crayfish with the elk back up:

Another crit for 5dmg

Zoveldra Opportunity Attack: Unarmed Strike to hit 23, damage 8 force

The monk dodges the sea creature's blow. Seeing the crayfish trying to drag the beasts under, she aims a thorny tendril to try to pull up the one grappling the snake.

Action: thorn whip to hit 12

Bonus Action: unarmed strike on the crayfish Erix pulled up. To hit 21, damage 9 force.

Crayfish 2 STR Save: 18