(Will run Eskil for round 1)

Eskil slips past Aubrey, relying on his small size to squeeze past in the narrow boat.  Aubrey hears him whisper something about 'let nature's life flow through him' and feels the wounds to his arm stiffen up and then begin to close and heal.  Bonus action Healing Word for 6

Eskil moves on to the collapsed form of Malia.  "Come now... we did not save you just to lose you now...".  He clamps his hands down over the wound on her shoulder and spends his action attemtping to stabilize her.  Medicine DC 10: 21

(ROUND TWO:  Leelan is up - need a CON save... Guilim too...  then the beast.)

Malia is stable!

"Thanks Eskil, take care of Malia for me." Aubrey whispers to the halfling, still scanning the waters for any reapperance of the many-headed beast.

Guilim will move to the pole and attempt to follow after the creature at the slow pace of the small boat.

Con save: 17

Getting hit by the strange green fluid, Leelan grunts in pain as the liquid burns in contact to his skin. He does manage however to shake off the toxicity of the projectile.

As the creature attempts to dive back down, the gnome pulls out a dagger and swipes at the beast.

Attack of Opportunity: 7 Damage: 4

However, it fails to make contact and he only hits the air.

Anticipating the monster will probably come back for seconds, Leelan tries to think of a way to draw it out for his companions to take it down. He suddenly has an idea. Using a simple magic trick, he conjures an illusion of another lobster cage on the water filled to the brim with bait. He hopes it will lure out the many-headed creature will fall for it.

(Casting minor illusion to create an image of a lobster cage filled with bait between the boat and the purplified creature).

Beneath the waves, the glowing purple shape completes a long, arcing turn through the water and then assume a collision course with the boat Guilim is poling in pursuit.  The creature is going to ram the boat!

... But then... it pull up short and turns in the water.  A waves splashed over the bow and into Guilim, soaking him but doing no harm.  On the surface along side the creature, something has caught its attention.  A lobster trap has appeared as if from nowhere, stuffed with a lobster nearly five feet long!

The beast rises up from the water, breaching the surface.  The good news:  some of the opportunity attacks must have found home, because now there are two bloody stumps and only three of its original five heads remain.  The bad news: from those stumps, four new heads have grown!  The beast now carries SEVEN heads!

(Aubrey's dissonant whispers held attack is triggered!)

The original surviving heads are all trained on the succulent lobster, and lash out at the trap.  But they find only air, and the three heads hiss in fury.  The four new heads remain locked on the enemy... the people who have harmed it and killed the old heads.

Out of melee range, the four heads spit gobs of poison at the boats.

One targets Guilim: Attack: 16 Damage: 4.  If it hits, a CON save DC 12 is necessary or you are poisoned until the end of your next turn.

One targets Aubrey: Attack: 9 Damage: 2.  If it hits, a CON save DC 12 is necessary or you are poisoned until the end of your next turn.

One targets Rohuzar: Attack: 14 Damage: 3.  If it hits, a CON save DC 12 is necessary or you are poisoned until the end of your next turn.

One targets Leelan:  Attack: 16 Damage: 4.  If it hits, a CON save DC 12 is necessary or you are poisoned until the end of your next turn.

(Eskil, Aubrey and Grugga are up for round two... then back around to Leelan, Rohuzar and Guilim for round 3.)

Dissonant Whispers Beast WIS save: 8  Damage:  9

Once Aubrey's spell sinks in, the creatures heads all howl with pain and fear, and it uses its reaction to swim off to the northwest, remaining on the surface.

Rohuzar con save 17 

Having no spells and only a dagger, Grugga will spend his turn grabbing Malia and moving her toward the other boat where it's hopefully safer. He'll also cry out to his allies, "Shaemus said this thing was afraid of fire, remember? Did anyone bring torches? Please tell me someone brought torches!" Grugga, of course, left all his torches back at the manor.

(OK.  Upon reconsideration, Aubrey's dissonant whispers would have triggered as soon as the beast broke the surface.  Nex, it would have failed its save and moved away per the effects of the spell.  So its attacks on the lobster trap never occur.  So the beast doesn't know the lobster is a fake yet... 

It is not in full cover - the wood debris on the map is all at or below water level and is a barrier to boats/movement but does not interrupt line of sight or provide cover.)

Post 346 is edited for corrections.

As the the blue serpent heads break surface Aubrey uses his talisman to assault their minds, wracking them with terrible pain and making the beast briefly flee away in fear. A moment later Aubrey dodges a gob of poison from one of the heads. His arm functioning again he reaches for his crossbow, takes a quick aim at the beast, trying to hit the body rather than one of the heads.
Light crossbow: 15 rolled with advantage because of faerie fire. Damage:7
He then tries to move over with Malia into the other pole boat as Guilim seems intent on getting closer to the many-headed beast.
"Rohuzar, should we try to fall back or press the attack?" Aubrey shouts quickly before taking as much cover as possible in the pole boat.

Grugga, since Malia is slight of build and you are standing on somewhat firm bottom, you are able to transfer her from one boat to the other with minimal difficulty (no roll required).

Aubrey, climbing into the water and back into the other boat can be accomplished this turn. 

(Going to bot Eskil for round 2)

Hearing Grugga's reminder about fire, Eskil remains at Guilim's side in that boat and casts produce flame, conjuring a mote of fire in his palm.  He hurls the ball of fire at the purple glowing beast.  Attack: 23 Damage: 8

... Though it is at the maximum range of Eskil's throw, the tiny ball of fire smacks into the creature very near to where Aubrey's bolt had just found home.  One of the necks shudders and then falls limply to the creature's side.  All the heads cry out in pain, and the twelve remaining eyes lock on to Eskil.

(We are back around to Leelan, Rohuzar and Guilim for round 3.  Aubrey and Malia are now in the front of Rohuzar's boat... Grugga still in the water... Guilim and Eskil in the front of the other boat in pursuit.  Monster looks mad.) 

Prolonged exposure to the creature’s venom had caused him to become sickly. He wasn’t so sure he could take another hit like that.

Nevertheless, seeing his companion fight on, the gnome gripped his shortbow and aimed it at the creature, now having sufficient distance to shoot it unhindered.

Attack: 19 Damage: 5

Con save: 11

 (poison disadvantage cancels out faerie fire advantage)

(ok for some reason my con save was moved from when I tried to edit a bit of text. The first roll was a 1. Still a fail anyway.)

Guilim will continue  to move the boat closer to monster so that others could have a better chance of using their weapons on it. If he has a chance he will ready himself to attack the creature if it gets within 10ft of the small boat.

(We still need Rohuzar's round 2 action... assuming a ranged attack with that javelin?  Also, the beast is going to move past Guilim and Eskil, attacking as it goes.  6 heads remain.  The attacks will be: 1 - Guilim; 2 - Eskil; 3 - Eskil; 4 - their boat; 5 - fake lobster trap; 6 - fake lobster trap.  The creature coming into range will trigger Guilim's readied attack - which uses his reaction so no AoO when the beast moves back out of range.  So I need both Rohuzar's Round 2 attack and Guilim's triggered readied attack as both of these would potentially hit the beast before it makes its attack and may kill more heads.  After I get those attack rolls, if any heads are killed.  I will roll 1d6 to determine which head dies and which attack never happens.)

Map showing where everyone is and where the beast stops at the end of its round 3 turn.

I think Rohuzar would ready an attack for regular distance, not wanting to waste the javelin. 

When it moves closer he throws!

Attack: 17 Damage: 7 

Guilim once more attempts to end the beast as be raises his halberd and swings it with all the strength his small body can muster.

Attack: 10 Damage: 5

20

(Guilim's attack would have still had advantage - unless I'm missing something.  But the DM is infallible.  So a miss becomes a hit.)

The beast's six heads hiss and snap as it surges back toward these hateful creatures that have invaded its hunting ground, challenging it for tasty, trapped lobsters.  As it reached the front of Guilim's vessel, the halfling attempts to cleave off another head with a powerful swing.  The neck moves just out of the way... Guilim cuts off only a patch of scales from its large body.  But then Rohuzar's javelin connects with the beast's side, eliciting another scream - but no more heads fall.  The creature moves past the boat in the direction of the illusionary lobster.  Along the way its jaws lunge out towards Guilim, Eskil - the one burning it with fire - and even their boat.  Once past, two more heads attempt to grasp the giant lobster - but there is nothing there!  Frustration!

Attack on Guilim: Attack: 5 Damage: 5

Attack on Eskil: 8 Damage: 6

Other Attack on Eskil: 21 Damage: 7

Attack on their Boat:  13 Damage: 6

For those sharp enough to notice something in the middle of desperate battle... perception DC 18:

10% chance here. Perception roll: 17