Gerrard:

"That's Captain Xondros!" says Gerrard.  "If she is the one controlling that sea monster, we must defeat her before it eats the ship!"

Kei 

Kei paces along the gunwale as he watches he's the bird thing take a dive. "Where is Kellen?" he grunts impatiently,  as if it's Kellens fault that the fight is still hundreds of feet away. When he spots the half elf fumbling his way back onto thw ship, Kei rushes to help him. "Get yourself together man" he shouts alongside an outstretched hand. 

Kellen

Stop attracting sea monsters! Kellen yells back, taking the outstretched hand. He offers a creative string of curses in few different languages as he plops on the deck.

I think it's high time we roll initiative!

Captain Xondros (injured, in the water, glaring at you from some 40 feet away): 1
Three nasty vulture things (one injured, floundering in water 40 feet away; two more high above, dive-bombing toward you): 18
Rurik (frantically fixing the tiller): 15
Tamra (climbing into the boat behind Kellen): 22 
Elwood's Warship (Several hundred feet away and closing from the northwest): 5
Giant sea serpent? (Several hundred feet away and closing from the east): 5
Kellen (finally back in the boat): __
Kei (helping Kellen climb in): __
Gerrard (shooting down massive flying horrors): __ 

Gerrard's Initiative:  25

Kellen's init: 16

Kei's initiative 19

Gerrard:

“How dare you attack MY city, minion of Iuz!” calls out Gerrrard in a commanding tone, his voice ringing with righteous anger.  With the foe now close, he switches to his magic arrows and attempts to perforate the demon worshipper before he can counterattack!

Longbow with Magic Arrow:  Attack: 17 Damage: 13

Longbow with Magic Arrow:  Attack: 17 Damage: 10

Longbow with Magic Arrow:  Attack: 26 Damage: 7

Unless he is sure Xondros is dead, he keeps shooting (action surge).

Longbow with Magic Arrow:  Attack: 18 Damage: 9

Longbow with normal arrow:  Attack: 14 Damage: 12

Longbow with normal arrow:  Attack: 21 Damage: 13

√ Gerrard (shooting down massive flying horrors): 25
√ Tamra (climbing into the boat behind Kellen): 22
   Kei (helping Kellen climb in): 19
   Three nasty vulture things (one injured, floundering in water 40 feet away; two more high above, dive-bombing toward you): 18
   Kellen (finally back in the boat): 16
   Rurik (frantically fixing the tiller): 15
   Elwood's Warship (Several hundred feet away and closing from the northwest): 5
   Giant sea serpent? (Several hundred feet away and closing from the east): 5
   Captain Xondros (injured, in the water, glaring at you from some 40 feet away): 1

Three arrows from Gerrard's double-volley strike the black-clad tiefling priest, who is clearly tougher than you remember her. She is nevertheless badly injured as she treads water near her floundering mount.

You see Tamra touch her neck in a rushed ritual, as a slight protective shimmer envelops her entire form. 

Kei may act next!

Kei 

Kei takes heed of Gerrard's statement. He looks squarely at the sodden Quartermaster and points at her bobbing form. A ball of sonic energy sweeps from his hand towards Xondros and her floundering mount. It explodes above the water in a ball of thunder 40ft across. Simultaneously,  his trusty tentacle rises up out of the water next to Xondros and slaps at her.

A - casts a 4th level Fireball with Transmuted metamagic to convert damage from Fire to Thunder. 29 Thunder damage on a failed DEX16 save. Half if saved.

Thunderball aimed to include both Xondros and the bird.

BA - Tentacle summon and attack. 29 for 8 Cold damage. 

If the Thunderball finishes Xondros then redirect the tentacle towards the bird.

The dark priest and her mount instinctively duck for cover under the surface: DC17 Dex Saves: 12, 19

The birdlike horror resurfaces; the priest does not. The tentacles tear into the screaming monstrosity, although the cold doesn't seem to bother it much (cold resistance.) It flaps its putrid wings and flies toward you, screaming.

Everyone aboard (except Rurik, who is at the stern of the ship): Make a DC15 Constitution Saving Throw or take 13 Thunder damage and have the Stunned condition until the end of the monster's next turn.

Tamra: 6

The other two flying things level off 20 feet above you, both dropping a grey cloud that smells of fungus and death.
Everyone: Make two DC15 CON Saves, or become poisoned.

Tamra: 21, 6.

Kellen may act next!

Gerrard:

Constitution Saves:

26

8

roll]1d20+7[/roll]

(May use Indomitable if Stunned)

(Third roll)

11

Con save: 5

Other two:  5  11

Kellen

Kellen, unprepared for the attack after finally getting on the boat, is overwhelmed by the scream attack and can't move. The cloud envelopes him in poison.

Kei

CON saves 16, 20, 13

Kei 

Somehow able to shake off the unnatural scream, Kei dodges the first bout of bird gas but backs himself into and unfavourable position and inhales the stinking cloud from the second bird. He coughs and splutters as they fly overhead. 

Tamra and Kellen seize up, stunned (until the end of the winged monstrosities' next turn.)

In fits of coughing from the foul spores, everyone except Rurik is poisoned. (You have Disadvantage on attack rolls and ability checks. You may repeat your DC15 save at the end of each of your turns, ending the effect on yourself on a success.

The warship fires off a barrage of long-range weaponry at the flying horrors above.

Mangonel #1 Stone Attack vs. flying horror #2: 15. Hit: 27 bludgeoning damage.
Mangonel #2 Stone Attack vs. flying horror #3: 15. Hit: 40 bludgeoning damage.
Ballista Bolt Attack #1 vs. flying horror #2: 16. Hit: 18 piercing damage.
Ballista Bolt Attack #2 vs. flying horror #2: 9. Hit: 19 piercing damage.
Ballista Bolt Attack #3 vs. flying horror #3: 8. Hit: 16 piercing damage.

Gerrard and Kei (both poisoned) may act! (And afterward roll another DC15 CON Save.)

Gerrard:

Not breathing the best right now anyway, Gerrard coughs in the poisoned air (saves should have been 1 lower with Constitution damage.  Will fix this time).  Nevertheless, he sees the screeching vulture creature that had carried Xondros before him and tries to finish it off!

Longbow attack at disadvantage:  Attack: 16 Damage: 12

Longbow attack at disadvantage:  Attack: 12 Damage: 8

Longbow attack at disadvantage:  Attack: 14 Damage: 7

He then tries to shake off the poisoned effect:  Constitution Save including disease:  23