The horror in the middle of the pack falls to the walkway, now a small octopus.

Boko may act next. She can see the first, wounded Skum attacking Gerrard from where she stands, as well as the third one watching her (albeit at 3/4 cover.) She may run in front of Gerrard (35' of movement) to get into position for a melee attack, but that would require a DC13 Dex save to avoid falling.

Boko taps her tattoos on her arm, activating their eldritch nature (BA for eldritch maul, +1/+1, 15 foot reach) and thrusts her spear at the wounded skum, moving forward to get within 15 feet if she needs to.

Action

Attack: 19 Damage: 14 plus 4 force damage

BA

Eldritch Maul

Boko springs out of the doorway, carefully clinging to the edge of the heavily rusted crane. Inky tendrils launch from her arm toward the wounded skum, sticking it deeply with her spear.

The third skum leaps into the fray, helping the first (wounded) one to try to shove Gerrard off the edge. Gerrard: make a DC17 Athletics or Acrobatics check or be pushed off the crane.

"Intruders!" they shriek, telepathically. "Trespassers!"

Everyone may act (and Gerrard should make that contested roll!)

Gerrard's Acrobatics to avoid the big shove:  22

Gerrard:

Flying over the edge, Gerrard tries to catch the edge of the platform if he can as he falls past (if possible, maybe stabbing his dagger into the boards?)

Can Boko run forward and jump to catch up to Gerard mid-air, and then cast dimension door to bring them to the boat?

Or if that's not possible, can she dimension door to Gerrard, then BA wildshape to a giant eagle right under him and secure him?

(Just trying to get an action economy DM ruling on which is more feasible.)

Gerrard plummets off the top of the crane, with nothing to grab, windmilling toward the surface of the greasy, rancid mud flat some sixty feet below.

Boko is too late to try to catch Gerrard in her present form, so she teleports just below him, grabbing at the falling noble with her newly-transformed talons. (Make a DC10 DEX check to catch him.)

Kei, alone with two monsters and an octopus on the rickety crane, may act next.

Kei 

Gerrard falls, Boko disappears. It all happens so quick. He sees Bokobird reappear below his friend but the rescue attempt seems to fail. Instinctively Kei reaches out a hand towards Gerrard. A wisp of shadow slinks around the noble noticeably slowing his descent. 

Kei turns back to the scum at hand. His inky tendril lashes out at the scum Boko had skewered. Meanwhile an outstretched hand fires 3 dark crackling beams at the surviving scum.

R - casts Feather Fall to slow Gerrard

BA - Tentacle Attack: 20 Damage: 11

A - Eldritch Blast x 3 at the skewered scum and then onto scum #3 if skewered scum falls

Attack: 23 Damage: 11

Attack: 16 Damage: 14

Attack: 29 Damage: 17

Boko the Giant eagle dex check: natural 1

She tries to swoop around for another pass to get underneath the now slowly falling Gerrard. Assuming that’s her whole turn.

It takes all three beams of black crackling energy (one of which misses), but Kei manages to slay the first of the gooey skum.

The third skum, having successfully removed two of the trespassers, now tries to remove the third. It springs forward, its movement seemingly unimpeded by the treacherous condition of the rusted crane. Kei: make a DC23 Athletics or Acrobatics check or be pushed off the crane.

Meanwhile, Boko-Eagle and a flailing, plummeting Gerrard scramble about in mid-air, trying to stop his imminent impact into the foul mud pit below. Things look dire until Kei's spell takes effect. (Boko may easily pick up a slowly-falling Gerrard now, without rolling, if you still choose to.)

Everyone may act. Gerrard and Boko are fifty feet below the crane's upper walkway.

Kei 

Contesting acrobatics check 6

Boko the eagle scout ensures Gerrard is safe and, when she spots Kei falling, she sweeps under him as well, and then flaps up to take a position above the skums, perhaps 15 feet overhead, attacking, and then moving up higher if she has reason to suspect these skums can jump high or have big reach. 

(Does the eldritch maul still work in giant eagle form? I think the tattoos have still been there and should work so discount if not.)

Giant eagle eldritch maul beak attack: 13 Damage: 8 plus 5 force damage
Giant eagle eldritch maul talon attack: Attack: 22 Damage: 13 plus 6 force damage

Kei is pushed violently off the crane, and plummets toward the mud. (He already used his reaction.)

~ Top of the round.~

With a flash of his hand, Kei arrests most of his velocity with a second Feather Fall spell about 50 feet below. (New reaction.)

Boko manages to catch a second passenger, then begins the rapid flight back upward.

(That's everyone's turn, unless Gerrard has an action he can perform from below, out of sight of the skum.)

(Fortunately, the third skum stays out of sight and performs no visible action.) (But in fact it is moving and Dodging.)

~ Top of the next round ~

Boko flies up and over the crane walkway, strafing the skum with its strange elongated shadow appendages. (Boko: Make two Disadvantage rolls for your rolls above.)

Gerrard and Kei may act as well, from 10-15 or so feet over the Dodging skum. (Kei has spent one reaction already this turn. )

Gerrard:

As he falls, Gerrard sheathes his weapons the better to cushion his landing and/or grab onto the oncoming Bokoeagle.  Recovering a bit of dignity and his balance, Gerrard pulls out his bow as they soar above the remaining Skum and he shoots at it, hoping to distract it from its dodging efforts.

Longbow Attack at Disadvantage:   Attack: 14 Damage: 13

Longbow Attack at Disadvantage:   Attack: 13 Damage: 6

Longbow Attack at Disadvantage:   Attack: 10 Damage: 13

If one hits, it distracts the Skum and deals 7 extra damage, removing disadvantage from the next person’s first attack.

Boko disadv roll 1: 20

Boko disadv roll 2: 25

Boko's oddly stretched beak and talons strike the gooey horror, as do two of Gerrard's three arrows, as the trio flies by. Cowering at the edge of the plank walk to the ship, the thing is pretty badly wounded.

Kei is up next! (No disadvantage on first attack, no spell restrictions!)

Kei 

As Kei dangles from Bokobird's talons he watches as Gerrard somehow not only fires a volley of arrows at the skum but actually hits the thing. He takes the momentary distraction and exploits it firing a series of projectiles of his own, only they're arcane in nature.

M - hangs limply like a caught rabbit in the eagles claws

A - casts EB at the skum (1 without disadvantage, 2 at disadvantage)

Attack: 25 Damage: 9

Attack: 16 Damage: 11

Attack: 11 Damage: 8

BA - Tentacle attack if allowed (at DA) Attack: 24 Damage: 11

Kei sprays the crane walkway with beams of crackling black energy, hitting it with one, then catches the thing with the brunt of his summoned tentacle. 

The badly-injured creature springs into the air on its extremely powerful - if slimy and translucent - legs, flailing its trident wildly at Kei.

Trident Attack #1 vs. Kei: 14. Hit: 5 piercing damage.
Trident Attack #2 vs. Kei: 13. Hit: 6 piercing damage.
Mind-Breaking Touch vs. Kei: 14. Hit: 15 psychic damage, and Kei has disadvantage on Wisdom saving throws until the end of the skum’s next turn.

Then, the skum falls back to the walkway, before scrambling across the precarious crude bridge to the boat, fumbling with a grate in the hull that belches steam. (No opportunity attacks due to gravity.)

Everyone may act!

Gerrard:

Maybe not the most honorable fight he has ever participated in, but with the Skum busy with the grate Gerrard sends another trio of arrows at its exposed back!

Longbow:  Attack: 15 Damage: 13

Longbow:  Attack: 22 Damage: Unable to parse dice roll.

Longbow:  Attack: 29 Damage: 14

Gerrard, still clinging to Boko-Eagle, skewers the skum with three more arrows. It slumps to the deck of the suspended ship next to the steamy grate, quite dead.

Combat is over; everyone may act.