Ah good, Jorin needn't dunk his head then...

Jorin will leap off the raft, weapons drawn, and ready an attack action triggered by an enemy coming into his reach.  "Quillary, what did you see??"

Attacks in spoiler

Quillary will heft her shield and back up, peering under the raft. (Active Action-spending Perception check, to be followed by free verbal description of what she sees (if anything): 15)

...space saved for possible use of Knowledge From A Past Life racial lineage feature (Reborn)... yes, please add 2 to the roll above.

Alerted by Quillary and then Jorin's reactions, Hildigrim readies an arrow and will fire it at any target he sees.

Quillary peeks underwater.
She sees one chuul underneath the raft, which was dragging the raft to the northeast. Its posture has changed, and it is getting ready to push up instead of pulling sideways.
Maybe in her past life she was ambushed, but Quillary looks towards the northern side passages. A second chuul is moving towards the raft from that location.

With Quillary's warning to the others, you are somewhat prepared for the attack.

The first chuul tries to lift and flip the raft, towards Q and J.
(I don't know how to handle this by the rules, so to start I'll make it an Athletics check contested by Drusk's  passive 13.)
Chuul lifting the raft: 5  (fails)

The second chuul snaps both of its pincers at Hildigrim.

• Attack1: 11 Damage: 11  (misses)
• Attack2: 12 Damage: 11  (misses)

(OOC: Ugh, those rolls...)
The first chuul tried to lift and flip the raft, but Drusk was able to use his pole to keep the raft stabilized.
The second chuul snaps its claws at Hildigrim, but the halfling nimbly sidesteps the attacks.

Pascal takes a few steps away from the raft and the chuuls, then shoots a flame bolt at the 2nd chuul (that was attacking H).

• Attack: 17 Damage: 16  (hits)

New round

Everyone except for Pascal can go. (Make suggestions for Drusk and Pascal, if you think of something).
Hildigrim gets a bow attack from the previous round.
Both chuuls are at the surface.

Jorin leaps off the raft and attacks the Chuul who tried to snip at Hildigrim.

• Bonus Action: R A G E at the one who attacks his smallest friend
• Movement: Dramatic leap attack
• Action: Battleaxes!
  • First Attack: 30 Damage: 20
  • Second Attack: 30 Damage: 15
  • +7 radiant damage on the first attack that lands

Edit - holy dang, here's another 3 of critical radiant damage

The second chuul snaps its claws at Hildigrim, but the halfling nimbly sidesteps the attacks. In response, and with a little bit of an involuntary squee, Hildi releases his held arrow at point-blank range.

Attack: 17 Damage: 6

Hildigrim jumps away (disengage) from the creature, moves 20 feet (in a direction that will still allow him to still see the creature) and fires another arrow.

Attack: 20 Damage: 7

Sneak attack: 11

(Uh, wow. Maybe Quillary should just stand back and perceive things every encounter! Those damage rolls, guys...)

Quillary stays where she is, but crooks a finger to bring the cold of the grave to the same chuul everyone else has been hitting. Her spores swirl angrily, but she is away from the enemy. If one comes within 10' they swarm it.

Action: Cast Chill Touch at the chuul. 20 to hit, for 7 necrotic damage.

Reaction: if one of the chuul gets within 10' her Halo of Spores will do 3 necrotic damage unless it makes a Con Save vs DC 15.

(Sorry for the delay.)

(Hildigrim's arrow from the previous round hits with a 19. The chuuls are using their 10' reach to avoid triggering Jorin's readied attack, but that also means they aren't close enough to give disadvantage on his archery.)

Chuul2 is no more, with Quillary's spectral hand finishing off the work done by Pascal (16 damage), Jorin (two criticals for 45) and Hildigrim (two arrows for 33).

Chuul1 CON save vs. spore halo DC 15: 13  (pass, resists spore damage)

Drusk will swing his mace at Chuul1. Attack: 15 Damage: 10  (hits)

Chuul1 stops trying to flip the raft. It moves to 10' from Hildigrim and attacks with both pincers.

• Attack1: 20 Damage: 11  (misses)
• Attack2: 21 Damage: 8  (hits)

Hildigrim dodges the first pincer, but is hit by the second. He is now grappled. The chuul dives back underwater, dragging the halfling with it. It swims 20' to the northeast, towards those side passages. (Since it is Large and halflings are Small, it can move at normal speed while carrying Hildigrim.)

Pascal runs after the fleeing chuul, water walking to stay within 10' of it, and tries a Ray of Frost. (It doesn't do as much damage as a fire bolt, but if it hits it will slow down the chuul.)

• Attack: 15 Damage: 14  (hits, the chuul swim speed is 10' slower)

Status update

One of the chuuls is chum.
The other one got Hildigrim with its claw (10 damage), dragged him underwater (he is holding his breath), and swam to the mouth of the side passages to the north.

That chuul has taken 16 damage from Drusk and Pascal. Its swim speed is 10' slower due to ray of frost.
Pascal is 10' south of the chuul & Hildigrim. The rest of the party is 25' to the south.

Hildigrim can use his action to try to break the grapple with Athletics or Acrobatics vs. DC 14. (His move speed is 0, but water walking will bring him to the river surface if he escapes). Or he can attack, or try a spell (which will use up his air if the spell has a verbal component.)

Melee/ranged attacks underwater will be with disadvantage (except for daggers).
(You have normal speed at the surface with water walking. If you are overhead, to get in melee range will require 10' of movement, to dive 5' below the river surface.)

Hildigrim, Jorin, Quillary, and Drusk are up.

Jorin shouts after Hildigrim and, at first, looks to dive below the water in pursuit, but then runs across the water until he is over-top of the Chuul and dives down after him from there.  He recklessly swings his blades in the water at the creature

• Movement: 40ft of movement to get within melee range of the Chuul.  If this isn't possible, ignore what's below and Jorin will dash to close the distance
• Action: Reckless Battleaxe attacks
  • Attack: 25 Damage: 11
  • Attack: 23 Damage: 12
• Bonus Action: Moonblade attack
  • Attack: 23 Damage: 16
• 7 of radiant damage to the first attack that lands

Quillary pads along the water’s surface until she is within sight again of the chuul, and once again points and crooks her finger, calling on the chill of the grave. Her spores swirl above the water, waiting for a target.

Action: cast Chill Touch again. 8 to hit, for 10 necrotic damage.

Hildigrim forces himself to calm down as the creature start swimming away with him. He squints against the murky water and flexes various muscles, analyzing how he is pinned. When he thinks he's found a chink in the creature's proverbial armor, he makes his move.

Acrobatics DC14: 25

He can't quite get free!

Chuuls are AC 16, so all 3 of Jorin's attacks hits for 42 damage. (He moved 25' to get over the chuul, then frantically thrashed in the water, fighting the buoyancy of Water Walk to get close enough to attack.)

Quillary's spectral hand is off target due to the slime and muck in the river. She moves within 15' of the chuul, so her spore attack will activate. CON save vs. DC 15: 6  (fails; Quillary can roll damage next turn)

Hildigrim struggles, but is still grappled. (And holding his breath.)

Drusk moves to keep pace with the others, leaving the raft to slowly float downstream. He casts Bless on Hildigrim, Jorin, and Quillary.

The chuul is slowed to 20' by the ray of frost. It will use its action to charge, letting it swim a total of 40'. (Jorin will get an AoO.) The pincers are occupied in helping it to swim/move, but its mouth tentacles stab at Hildigrim.
Hildigrim should roll a CON poison save vs. DC 13, o be paralyzed. (If paralyzed, he gets to roll again at the end of his turn.)

Pascal will run along the river surface, and shoot another ray of frost at the chuul.

• Attack: 24 Damage: 8  (hits)

(Put the map in a spoiler to save space. View it as much as you like. Pascal and Hildigrim see the new area, but the others will next round.)

New round...

The chuul is below half health, but still has a bunch of HP left. (But not if Jorin keeps 3-hitting it.)
Hildigrim, Jorin, and Quillary have been Blessed (add d4 for to-hit and saves).
Hildigrim needs to roll a CON save vs poison, or be paralyzed. He is still grappled by the chuul's claw.
Jorin got a free swing (AoO) from last round. He loses 20' of movement this round floating to the river surface and getting back to his feet.
Quillary gets to roll for halo spore damage from last round.

Hildigrim, Jorin, and Quillary are up (and Drusk too).

(Spore damage for last round: 5)

AoO from the Chuul moving

• Attack: 21 + 1 to the hit roll from bless
• Damage: 12  axe damage + 6 radiant

CON save DC13: 22

..

Seriously?

(Jorin hits with the AoO, but already used Divine Fury that round (does not get the extra radiant damage).

Hildigrim has Bless, so roll that d4 also.)

Oh yeah! Bless: 4

That's more like it.

Hildigrim is just able to slap aside the tentacles before any poison is injected.

Drusk will use his action to charge (on top of his normal movement). With 60' to use, he will end up 20' NE of the red X (map in post 4862). The chuul is still 10' underwater, so he uses his bonus action for Spiritual Weapon.

• Attack: 15 Damage: 11

Hildigrim, Jorin, and Quillary are up.