19 Initiative.  I'm standing at the top of the stairs that separates the upper and lower decks.  Mary, could you be a doll and assist us for whatever threat is imminent?  Yea, and the rest of you too.  If this boat goes down, we all go down.

Initiative: 9

Absinthe would have been below deck attempting to teach some of the sailors how to brew his ale. 

Perception 21   passive 23

Initiative.     17    

Top deck mid maybe a little closer to the aft

"Awwww great big gangly bollocks, what shite luck!" Fjolnir shouts out at no one in particular, from his perch half-way up the rigging.
Initiative 14

Init 12

Dusty is below deck, holding his guts by the cargo.

Currently, all of you on the top deck see a multitude of large shapes surrounding the boat underwater. They appear fish-like in their movement, yet they seem to have arm-like limbs or fins toward the front. It's impossible to make out any more details through the waves, but they are keeping pace with the boat. Absinthe and Dustfinger, as well as the rest of the crew below deck, hears the captain's call to arms.

10 14 19

Notes: The rigging can be climbed without making any kind of check, it is around the mast and will allow you to cross the center of the ship to get to the other side without going around.
You can make a ranged attack against any one of the creatures if you're above deck and against that side of the ship, but since they're a few feet underwater it will be at disadvantage.
There are 9 dark shapes underwater on either side of the boat.
Let me know if you would like to start from a different location, you can be anywhere on the boat.
There are 2 people operating each boat-mounted weapon. If there is ever not 2 people on a weapon, it will not be able to fire. Anyone may take the place of a weapon operator if they fall, but for the most part the other guards lined up along the sides of the deck will take care of that.
Mary, Garrett, and Gearbox are on the map and will act on their turns. Anyone, on their turn, can call out what you want them to do as a free action, otherwise they will act to the best of their abilities. However, now that they are aiding the party they can be targeted.
Captain Miggetter, His first mate and bosun are on the map. The captain will be calling orders and keeping the ship on course, the other two will act as his bodyguards to make sure he can continue doing that.

Absinthe and Dirk are up. 

21- Absinthe
19- Dirk
19- Gearbox
16- Captain and Mangonels
14- Mary
14- Fjolnir
12- Dustfinger
10- Garrett
9- First Mate and Bosun
8- Az Emberi
8- Guards
8- ???
4- Ballistae

I run to the left side of the boat.  Hey, what are you up to down there!?  You're spooking the land lubbers!  I call out to the creatures below, hold action firebolt if we are attacked on nearest target.  11

Absinthe makes his way above deck and finds a spot between some of the sailors on the port side and waits to see if anything starts climbing up the side of the ship. 

Round 1: What the Folk?

Absinthe runs up the stairs at the sound of the Captain's battle call and leaps from the stairs to the left side of the ship between two guards to see the shapes in the water. He shrugs and readies a dart in case one pops up nearby.

Dirk does the same and tries the vocal approach before attacking, but readies a firebolt just in case.

Miggetter sees Absinthe and Dirk quickly respond to the potential threat and yells, "Ready, don't fire until they surface or show aggression, then let 'em have it!" He keeps the current heading and speed.

The Mangonel operators get ready to fire when they see anything.

Mary readies an Eldritch Blast.

Fjolnir and Dustfinger, you're up! Az Emberi can go ahead and act, too.

Fjolnir readies a spell for the slightest sign of aggression- whispering a Hail of Thorns incantation on a nocked arrow.

Dusty tries to make his way topside, clutching his pact weapon in one hand, the energies of a held blast collecting in the other. 

Fjolnir and Dustfinger both ready ranged attacks. (Dustfinger, pick which side of the ship you're going to.)

Garrett, the First Mate and Bosun join in holding for the first sign of aggression.

Az Emberi is up.

21- Absinthe
19- Dirk
19- Gearbox
16- Captain and Mangonels
14- Mary
14- Fjolnir
12- Dustfinger
10- Garrett
9- First Mate and Bosun
8- Az Emberi
8- Guards
8- ???
4- Ballistae

Ba stones ready throw for first show of agression.

 The guards ready spears at the shapes below the waves.

With a burst of sea spray, several of the dark shapes leap out of the water and while keeping pace with the ship, crash back into the water, their torsos above the water-line. The creatures look like giant merfolk, but warped and further removed from a humanoid race. The captain yells, "Ah Shite! Merrow! Fight fer yer lives!"

Their eyes burn with pure hatred and bestial hunger as they scan the deck for targets. There are four of them that surfaced, and each one rears back their throwing arm with a hooked javelin ready to fly. At this point, before any of them attack, you, your hired guides, the guards, and the manned weapons unleash upon the violent looking creatures that just showed their faces:

Absinthe throws a dart, even though it's at long range: Attack: 10 Damage: 5 
He still manages to expertly connect! (I removed a dart from your inventory, you're probably not getting that one back.)

Dirk shoots a firebolt but it fizzles harmlessly on the Merrow's wet hide. (It was a miss, they're not immune to fire, just adding some flavor.)

Gearbox is at long range for a javelin, but lobs one at #12: Attack: 12 Damage: 2
It misses.

The two Mangonels at the ship's rear take aim at #2 and #4. (I just read up on these and apparently it takes 2 actions to aim these weapons, so both of the guards operating them used their actions to aim them and they can now fire at creatures around the front half of the ship, since they cannot fire at anything within 60 feet, either.

Mary releases two eldritch blast beams at #12: Attack: 16 Damage: 6 / Attack: 14 Damage: 7
The first goes wide but the second singes its arm.

Fjolnir fires at #11 (I'm assuming you cast Hail of Thorns as your bonus and readied the attack instead of readying the spell.): Attack: 25 Damage: 7 If hit, the target takes an additional 10 piercing. If missed, you maintain concentration on Hail of Thorns.
The arrow sinks into the creature and bursts into tiny shards, doing 18 damage total, but not affecting anything else as it's basically alone.

Dustfinger, like Mary, releases two eldritch blast beams, but at #11: Attack: 14 Damage: 9 / Attack: 15 Damage: 1
The first one strikes it in the chest but the second glances off a well-placed shell.

Garrett releases a slew of taunts and insults against Merrow #11's mother. (Vicious Mockery) WIS Save vs DC 13: 13 The words seem to not hurt the merrow.

First Mate and Bosun fire longbows at 11 and 12 respectively: Attack: 10 Damage: 7 / Attack: 24 Damage: 7 
The Bosun hits 12 in the side.

Az Emberi whips a stone at #11: Attack: 15 Damage: 5 
But it passes by the creature and skips twice before sinking harmlessly below the surface.

The 20 Guards and the 4 Merrow simultaneously throw spears and hooked javelins: (good luck to me not ******* this one up.)
x4 vs #11: Attack: 15 Damage: 3 / Attack: 18 Damage: 5 / Attack: 21 Damage: 6 / Attack: 9 Damage: 6 2 hit for 5 damage.
x6 @ Disadvantage vs #11: Attack: 20 Damage: 3 / Attack: 7 Damage: 4 / Attack: 17 Damage: 6 / Attack: 12 Damage: 5 / Attack: 5 Damage: 3 / Attack: 13 Damage: 4 2 hit for 8 damage.
x5 vs #12: Attack: 12 Damage: 6 / 14 Damage: 7 / 15 Damage: 5 / 20 Damage: 6 / 6 Damage: 6 3 hit for 17 damage.
x3 @ Disadvantage vs #12: Attack: 14 Damage: 5 / Attack: 8 Damage: 4 / Attack: 17 Damage: 4 1 hit for 4 damage.
x2 @ Disadvantage vs #4: Attack: 11 Damage: 3 / Attack: 5 Damage: 2 none hit.

As the merrow lob their javelins, you realize they are attached to a line. They are throwing harpoons.
Merrow attacks: #2: Attack: 21 Damage: 13 The harpoon bounces off the side of the ship.
#4: Attack: 15 Damage: 12 This guard dodges but the merrow reels its harpoon back.
#11: Attack: 23 Damage: 13 The harpoon clangs on the guards shield and falls into the water.
#12: Attack: 11 Damage: 14 This harpoon skewers the guard adjacent to Mary. The merrow yanks on the line. 5 vs 21 The guard uses the railing to stay on board the ship, nearly falling overboard. The harpoon is through his torso and he is screaming in pain.
#2, 4, and 11 sink back beneath the waves (giving disadvantage to any further attacks). Merrow #12 stays above to keep his hold on his "catch."

The two Ballistae on the right side of the ship aim and hold ready to fire at #2 and #4.
The two on the left aim and hold ready to fire at #1 and #3 should they surface soon.

Absinthe and Dirk are up!

21- Absinthe
19- Dirk
19- Gearbox
16- Captain Miggetter
16- Mangonels (Aiming at #2 and #4)
14- Mary
14- Fjolnir
12- Dustfinger
10- Garrett
9- First Mate and Bosun
8- Az Emberi
8- Guards (1@ 5 HP, currently skewered)
8- Merrow x18 AC: between 13 and 15 / HP: at least 44
#11- (Taken 44 damage, looks rough)
#12- (Taken 29 damage, looks hurt)
4- Ballistae holding attack actions for #1, 2, 3 and 4.

I move up the boat close to Fjolnir, Ready an L3 chaos bolt for when the next few Mer-billies surface.  Tides of chaos,8   13  15    (Forgot to click advantage 18) (Damn it forgot to add the modifier too +7) THUNDER.     11 wild magic table thingy..... 19

Absinthe moves diagonally SE two squares and S six squares to put himself just NW of the man that was harpooned. (Took my 40ft move to get there) In an attempt to break the rope. (I'll roll a strength check but don't know what I need)

Strength: 18

What the Folk? Round 2

Absinthe (is trying to stand in mid-air, will edit when he updates his post.)

Dirk readies a nasty ball of chaotic energy for the next thing to pop its head up. Cleaned up: 21 to hit, 20 Thunder damage. Nice! Something is taking that hit, do you have a preference which #? Also, roll another d20 to see if you get a 1 and have to roll on the Magic Surge table :D

Gearbox pulls out another javelin but waits for something closer to target.

Captain Miggetter stays the course and yells, "Same plan, ready a volley, don't wait fer my command, fire when you see 'em!"

The Mangonels Are aimed at the shadowy shapes of #2 and 4, and are ready to release as soon as they surface.

Mary casts Hex on #12, giving it disadvantage on STR Checks, and blasts it eldritchly. Attack: 22 Damage: Unable to parse dice roll. / Attack: 21 Damage: Unable to parse dice roll. 
(messed up the damage, tried to add 1d10 and 1d6 on the damage line, not sure how.) One beam strikes the creature for 8 Force and 6 Necrotic.

Fjolnir and Dustfinger are up.

21- Absinthe
19- Dirk
19- Gearbox
16- Captain Miggetter
16- Mangonels (Aiming at #2 and #4)
14- Mary
14- Fjolnir
12- Dustfinger
10- Garrett
9- First Mate and Bosun
8- Az Emberi
8- Guards (1@ 5 HP, currently skewered)
8- Merrow x18 AC: between 13 and 15 / HP: at least 44
#11- (Taken 44 damage, looks rough)
#12- (Taken 37 damage, looks hurt, Hex: (D) on STR Checks)
4- Ballistae holding attack actions for #1, 2, 3 and 4.

Dustfinger utters an arcane obscenity complete with a bird and keeps up the aggression on the same merrow he just struck, with two more beams of disco rage.

BA casting Hex, cursing Dex

EB 1 18 Damage 6 + Hex 3

EB 2 17 Damage 2 + Hex 4 

Disad 119

Disad 2 10

"We shoulda traded repellent instead of weapons with the mer!" Fjolnir eyes the surfaced assfacefish and concentrates a hunters mark on it, firing one arrow right away, and immediately following with another.

Longbow  att 16 dmg 6     att 14 dmg 10     Mark 1

Fjolnir fires at the Merrow struggling to pull down the crewman, (You get Hunter's Mark for each hit while it's active, so you do an extra 3 damage! LOL) Both arrows pierce the center mass of the creature, and it lolls underneath the waves, sinking and letting go of the rope on the harpoon.

Dustfinger sends a blast into the water and despite the inability to see it clearly, sends it sinking to the depths.

Garrett decides to cast something at one of the creatures below and crumples a tiny tart in his hand while waving a feather... Merrow #17 Makes a CHA Save DC 13: 16 (almost!) The Merrow stops moving along with the ship and bubbles erupt from directly above him as hideous laughter is heard. Then Garrett strums a note, giving Dustfinger a d6 Bardic inspiration.

The First Mate and Bosun ready attacks for more to surface.

Az Emberi is up.

21- Absinthe
19- Dirk (Holding Chaos Bolt)
19- Gearbox (holding attack)
16- Captain Miggetter
16- Mangonels (Holding for #2 and #4)
14- Mary (C) Hex
14- Fjolnir
12- Dustfinger (1d6 Bardic Inspiration) (C) Hex
10- Garrett Spells: 1[X][][][] Bardic Inspiration: [X][][] (C) Hideous Laughter
9- First Mate and Bosun (Holding Attacks)
8- Az Emberi
8- Guards (1@ 5 HP, currently skewered)
8- Merrow x16 AC: between 13 and 15 / HP: at least 44
#11, 12- Dead
#17) Hideous Laughter (Incapacitated, save at the end of turn or damage)
4- Ballistae holding attack actions for #1, 2, 3 and 4.