DM rolls:

Jorin and Drusk continue guiding the reft downriver. The aboleth (illusion) keeps pace, staying 55' back. Quillary keeps an eye behind the raft, while Pascal and Hildigrim watch ahead.

When you are 20' SW of the next two water passages (on the right edge of the previous map), Hildigrim gets more nervous than he already is. Even with the river flowing and the muck swirling about, Hildi thought he saw excess waves at the mouth of those two passages.

(Hildi's OOC reaction: link to YouTube video )

 (ha, i just picked up Aliens: Fireteam)

Jorin would like to ready an attack with his battleaxe should something leap out of the water into chopping range.

Quillary will cast Shillelagh (Bonus Action spell) on her club, and hoist her shield, standing next to Jorin (she believe she's tough like the fighters). If she has time, she will also Ready a hit with her club for if something leaps from the water.

Hildigrim readies an arrow and has Volo fly overhead.

Drusk continues pushing the raft with the pontoon pole, while the others are alert for combat.

The aboleth (illusion) closes the distance, until it is 40' behind the raft. Its tentacles move in a precise, rhythmic pattern.
You notice that the raft is moving slower, and northeast towards the 2 water passages, which are 15' away.

Drusk: "Something is moving the raft in that direction. I'm trying to move us southeast, but I can't get enough leverage with this pole (by myself)."

"There is an easy answer to that," Quillary says, stepping off the raft to stand on the water "Let the creature take the raft where it will."

Jorin is about to lower his weapon and help Drusk pole when Quillary steps off the raft.

"Quillary, how long do we have before this ensorcelment wears off?"

Quote from Guiltysparc >>

"Quillary, how long do we have before this ensorcelment wears off?"

(Let's say that Hildigrim is about to lose Detect Magic soon, and Water Walking has another 50 minutes.)

”Enough time to finish this gahlrahk-achk, perhaps an hour?” she responds. Apparently she only knows how to curse in Primordial?

Quote from Guiltysparc >>

Jorin is about to lower his weapon and help Drusk pole when Quillary steps off the raft.

(Does Jorin help push the raft?)

Pascal has been patient enough, and decides to launch a fire bolt at the aboleth (illusion). On impact, the illusion disappears. Satisfied, Pascal blows a small amount of smoke from his index finger/claw.

Drusk: "Whatever is pulling the raft towards those northern water passages is still as strong as before." The raft is still inching to the northeast, despite the half-orc's best efforts. He is looking at Quillary standing on the water, and then the others, to see if they want to abandon the craft or not. (Or what else they want to do about the current situation.)

Hey, DM, does Hildigrim have any idea how close the illusion-caster would need to be?

Quote from DirtyDogP >>

Hey, DM, does Hildigrim have any idea how close the illusion-caster would need to be?

(Hildigrim knows this is an unusual situation, and how the aboleth creates an illusion is not like typical spell-casters.
For example, it used the "senses" of illusion, like Hildigrim does through Volo. And use telepathy through the illusion.)

Jorin will continue helping pole the raft since Pascal nuked the illusion.

Quillary will pace along beside or behind the raft (if the passage is too narrow).

After Jorin joins in, the raft is now inching to the southeast away from the side passages, instead of towards it. But the progress is very slow. (30 seconds to move 1')

The river at this location is 30' wide, so Quillary has plenty of space.

With Pascal standing next to Hildigrim: "It occurs to me that you've been watching with your Detect Magic. You see the Water Walking on all of us, and you saw the illusion magic when you got within range. But you haven't seen any magic on the raft. If there isn't magic affecting the raft or the water around it, then why are Jorin and Drusk having so much trouble pushing this raft?"

Hildigrim has a moment to think, but the group hears a splash near those northern side passages. You don't see what caused it, but the swirl of sludgy water suggests something large moved in that area.

Here's a roll to try to answer Pascal's question (I'll let you decide what modifier to add): 17

Quote from DirtyDogP >>

Here's a roll to try to answer Pascal's question (I'll let you decide what modifier to add): 10

Hildigrim isn't sure. (His anxiety and having Volo on high alert has him distracted.)

But their discussion is loud enough, that maybe Jorin or Quillary might have an idea.

(OOC: No check rolls, please. Asking if the players have an idea. Or maybe not. Figured I'd give the party a chance to prepare, before what comes next.)

“Perhaps something is… pushing the raft from below?” Quillary looks at the slimy water with reluctance. “I do not really want to dip my head to look…”

Jorin sighs and then dunks his head in the water to get a look under the raft...axe in hand of course.

Quote from Mingofaust >>

“Perhaps something is… pushing the raft from below?” Quillary looks at the slimy water with reluctance. “I do not really want to dip my head to look…”

Quillary looks down at the slimy water under her boots, then under the reft next to her.
(OOC Q's reaction link to YT from 2:40-3:00)

Combat starts...
Everyone except for Pascal goes before your opponents. (Pascal is after them in the initiative order.)

Hildigrim, Jorin, and Quillary are up.
(Ready an action is always an option. An active Perception check would use up your action. Feel free to suggest what Drusk and Pascal should do, too.)

(EDIT: Guiltysparc posted while I wrote this. He can stay with an active Perception check, or post something different if desired.)