Guilim will listen to the various theories but keeps his thoughts to himself as he keeps an eye on the girl, keeping silent as while she was an elf she was not one of the hated ones that had forced his family from their homes. As they get closer to the last trap he will move to the edge of the boat, readying his halberd to stab down into the water.

“Hag. Hm. Ve must find it first. Now that you know what we are looking for, can you find signs of a lair?” 
(@dm: are we even capable of fighting a hag? I don’t know if we’re talking about something within our capacity to fight or if this is like fighting way out of our weight class)

As hypotheses are traded back and forth, the team slowly works their way closer to the final trap.  Out here, the sound of the surf crashing against the outer seawall coupled with the rain thrumming on the docks above nearly drown out your fellow's voices.  Some of the wooden columns out here show signs of damage.  A few are splintered, while others show signs of recent repair.  The wind is also able to reach deeper under the docks out here.  Rohuzar and Aubrey both suspect if they weren't controlling the boats, they would begin to drift east with the wind.

12

Still, despite all the noise, Aubrey hears another sound ahead in the water.  A splash.  A ripple, another splash.  Those with goggle lamps see a faint glimmer.  And then there they are... a pair of golden eyes, reflecting back the dim light.  Then another pair, both snapped around and locked on you.  Then they are gone.

The three with darkvision get a bit more of the picture.

After a few moments, as the party collects their breath, but before they can share any communication... SPLASH.  Something bobs up onto the surface 20 feet ahead.  After a startle, those with darkvision are able to tell it is the shredded carapace of a large freshwater lobster.  It bobs silently now on the surface.

For the moment... the waters are still... 

"What's going on...?" those in the boat with Malia hear her whisper.

Rohuzar pulls his battleaxe from his belt loop. 

“Silence now… must be hearing…” 

Rohuzar looks and listens, ready for an attack, especially if something comes for Malia. 

perception 24 

(Rohuzar's Nat20 is noted.  We'll wait a bit for others to act in this moment before I share the results.)

Grugga pulls his mask back down and slides into the waist deep water with his dagger drawn (ideally he doesn't make any more noise than the heavy rains to bother Rohuzar). He dunks his head head under the water and looks around in a 360 for any sign of those snake... eel... things.

My terrible perception: 18

Guilim readies his goggles to be able to see, moving to step closer to the water with his weapon pointed down to serve as an impromptu harpoon if needed.

Aubrey also readies his goggles, as well as his crossbow, sitting down again, using telekinesis to steady the pole boat if necessary, doing his best to spot the golden eyed creatures with the light from Grugga´s invention.
Perception:9

"Don´t worry Malia, this will all be over soon." He whispers with as steady voice as he can muster to the young elf beside him. Persuasion if needed to keep Malia calm:23

Rohuzar turns to face the water between the boats calling quickly “Something big here!” And raises his battleaxe high to bring it down on whatever it is as soon as it surfaces.

Aubrey does his best to keep Malia calm, while also keeping a lookout for danger.  He does a better job at the latter than the former, as he can see the girl's anxiety rising as she picks up on the nervous tension in the boats.  Meanwhile, he senses rather than sees something... a pressure in the water like a wave, but one he cannot see....

Grugga meanwhile slides over the side of the boat.  The water is a bit deeper than he'd anticipated, coming up to his navel.  He stands just to the west of the boat and lowers his face into the water.  The waves, surf, wind and rain have churned up the water.  It is murky and visibility is only 5-10 feet even with his darkvision.  What was that?  To his right, on the other side of the boat, there's a flash of movement, the writhing of a long, crocodilian tail.  Suddenly, Grugga cannot help but feel very exposed out here in the water....

Rohuzar sees what the others do not... his trained eyes catching the unnatural motion of the water, waves briefly turning the wrong direction, the surface swelling when it should be falling.  Something is coming at them fast.  It is right between the boats.  “Something big here!” he cries to alert his fellows, as he raises his axe high overhead, prepared to strike.

Guilim sits poised over the side, his halberd raised high like a harpoon.  He does not see the threat coming, but Rohuzar's warning is enough to trigger his instincts, and he has his own opportunity to strike....

Leelan and Eskil, at the rear of the boats, have the worst view and don't pick up on what is happening.  Luckily, Rohuzar's warning cry gives them a heartbeat to prepare and prevents them being taken by surprise when....

A large monster erupts from the water in between the two boats!  Five serpentine necks each ending with a snapping, sharp-toothed lizard-like head emerge from a central torso.  Huge, pale eyes dart back and forth as the heads select their targets from among you.  Two vestigial, almost flipper-like limbs hang from its side, and behind it the water is churned by a long, thick tail not unlike a crocodile's.  The entire body is covered in blue scales that shimmer like sapphires in the dim light of your headlamps.

Nature DC 10 or Arcana DC 14

 Aubrey 23  Grugga 9  Guilim 19  Leelan 9  Rohuzar 12  Eskil 6  Monster 15  Malia 14

(I am giving Rohuzar and Guilim the opportunity to get in a SURPRISE ROUND attack as they were warned and had their weapons ready.  Once that is resolved we will be in initiative as follows....)

• Leelan
• Rohuzar
• Guilim
• Monster
• Malia
• Eskil
• Aubrey
• Grugga

 (I need Rohuzar and Guilim to take their free shots... then I need the turns for Leelan, Rohuzar and Guilim, then we will see what Big Blue does first...)

Rohuzar brings his axe down on the creature as soon as he sees it appear

Attack: 16 Damage: 5 

Rohuzar is instead thrown off balance by the creature bursting up out of the water, but he’s quick to recover and swings again 

Attack: 14 Damage: 5 

Guilim grits his teeth and turns his weapon in hand, taking a step forward as he aims to stab the blade into the head of one of the creature.

Halberd: Attack: 16 Damage: 10

As the creature emerges from the water, Leelan attempts to ascertain its nature.

Nature check: 11

However, the sudden chaos prevents him from thinking straight. He then prepares to dodge any incoming attacks as he is not in a good position to shoot the monster. (Dodge action)

Grabbing the pole, he attempts to push the boat ever so slightly out of reach of the creature so that he is not in range, but Rohuzar can still fight.

(essentially, I’m trying to row the boat back 5ft. From my understanding, Rohuzar would still be in range for his attack, but not leelan)

Second Attack: Attack: 20 Damage: 10

As the beast erupts from the water, Rohuzar is waiting and brings down his axe with a mighty swing.  But his aim is off, and his swing nearly sends him tumbling.  He catches himself and attempts another slash.  This time he glances off the creature, but his lack of balance keeps him from getting any power behind the swing and the creatures natural scale armor saves it from any damage.

Guilim's lower center of gravity saves him from the same problem, and he savagely stabs downward at one of the heads, connecting once, and then again.  Blood sprays into the water and the neck closest to Guilim sags and falls, leaving only four heads remaining.

Leelan grabs the pole Rohuzar abandoned and pushes the boat five feet back, moving himself out of melee range but leaving Rohuzar within reach of the beast.

The four surviving heads snap and hiss, large pale eyes seeking targets.

One lashes out to bite Rohuzar:  Attack: 5 Damage: 4   In it's zeal, its snaps its jaws on its own dead neck, missing Rohuzar completely.

One attempts to bite Aubrey: Attack: 24 Damage: 8

Another head tries to snap its jaws on Malia: Attack: 21 Damage: 5   The jaws lock onto her shoulder and she his shaken violently before it releases her.  She cries out in pain and then drops, hanging half over the side, unconscious.

The last surviving head locks onto Leelan as he poles the boat away... and it spits a gob of green fluid at him: Attack: 18 Damage: 3.  If it hits, Leelan must make a CON savings throw DC 12 or be poisoned until the end of his next turn. (Edit - this should have been at disadvantage due to Dodge… extra attack roll for disadvantage… apply the lower… 8 )

After its attacks, the creature dives back below the surface.  You can all catch a glimpse of its scales below the surface as it moves down and out between the boats, remaining submerged and ending up somewhere near Q2.  (Those of you within melee range of the creature when it dove (Rohuzar, Guilim, Aubrey, Eskil... then Leelan as it moves into your range and away) can attempt an AoO.  The beast is underwater and AoO will be at disadvantage.)

Eskil, Aubrey, and Grugga are up this round... then Leelan, Rohuzar and Guilim are back up next round before the beast.

Malia:  9

(***** EDIT - Too many edits, I think.  The original rolls are all intact but the rolls I added are changing.  The first disadvantage attack on Leelan was a 17 so with the 18 that would still be a hit.  The first Malia death save rolled as a NAT 1 equal to two fails... I am going to stop touching this post now before other rolls start changing......)

Still somewhat distracted by the young elf Aubrey reacts to slow and the blue scaled beast savagely sinks it´s teeth into his arm, making him drop his crossbow. "Noo!" He quickly wriggles free but is distraught at the sight of Malia being shaken violently before dropping back into the boat. Gritting his teeth while bleeding profusely from his left arm he kneels down and tries to move the unconscious elf back into the boat, trying to get them both into cover if possible. (Move) He then scans the surface, ready to retaliate as soon as he catches sight of the beast again. (Assuming Aubrey can´t see the beast. Ready action to use Dissonant Whispers as soon as one of the heads breaks the surface within 60´)

Guilim curses as he sees the Elf being thrown about, moving to swing his halberd to get one last attack on the beast as it dives.

Attack: 20 Damage: 11

Watching the ripples of the beast move past him, Grugga pulls out an uncut, cylindrical shaped, lavender crystal from his pouch along with a small "hammer" mounted on a ring. He fits the ring around the crystal like he was putting it on a finger, pulls the hammer back until it makes a clicking noise, then throws it in an arc over where he thinks the strange beast is. Once it hits its apogee, the hammer fires, smashing the crystal into purple dust which rains over tiles O,P,Q,R, 1, 2, 3, and 4 (20 ft cube). (casts Faerie Fire Hydra makes a DC 13 dex save or becomes limned in purple light, rendering invisibility impossible, making him glow dim light in a 10 ft radius, and granting advantage on attacks against it. Also, the pier footing is now glowing purple (if you count it as an object) and sheds dim light in a 10 ft radius. Grugga is concentrating on this. Grugga has 0 spell slots left).

Then he uses his movement to move behind his boat, about where Rohuzar is (15 ft, max movement with difficult terrain), trying to take cover from any spit attacks

Monster DEX save 7

The area described by Grugga is now bathed in a purple glow... as is the full length of the submerged creature.

(For the moment, while underwater, attacks against it are at disadvantage + faerie fire advantage = no advantage or disadvantage... once it resurfaces, if Grugga's concentration is still up, all attacks will be at advantage.  Also, regardless of range all PC's can now see the creature in the water, darkvision or not).

(Waiting for Eskil... Aubrey has a ranged spelll attack readied... and then back around to Leelan, Rohuzar and Guilim... OH and Rohuzar can still take an AoO at disadvantage against the beast from when it moved)

Rohuzar tries to swing at the creature as it dives beneath the water 

Attack: 18 Damage: 12 

forgot disadvantage 19 

Rohuzar then spends his turn putting away his battleaxe and drawing a javelin