Belua's arrogance is misplaced. Kairouani's arrow arcs over the water and buries itself in his back, causing him to stumble forward and cry out in pain and anger.

Belua's interpreter glances at the nobleman with worry in his eyes. He licks his lips nervously, then nocks an arrow to his shortbow, draws, and looses towards Tsara. Tsara and Mitombo have not fallen prone, but are standing on the water's edge indecisively. Attack: 15 Damage: 3

The arrow strikes Tsara in the thigh and he yelps in pain.

Thea, your turn.

Thea Oltineas

Thea grits her teeth as still the bastard Belua didn't fall. She knocks another arrow and takes a shaky breath. If she didn't kill him soon, he'd be out of range and escape. Time to end this. She closes one eye, staring down the shaft of her arrow that the leader of the rotten gang aboard the boat. He would not escape. Then, she stops. What if he couldn't escape?

"Kill the oarsmen!" She calls to her companions, already sending an arrow flying into the midst of one of the rowing scoundrels. It whistles as it soars through the air, before imbedding in flesh.

Notes

Just gonna make another shortbow attack then drop prone again. If you'd allow it, I'd like to use my Bonus Action to Cunning Action hide in the sand dunes after shooting the arrow (Stealth: 22)

Short-Bow Attack: 24 Damage: Unable to parse dice roll.

Thea's arrow strikes the rear-most rower, the one who had tried to pull his mate from the water a few moments earlier. He grimaces in pain but keeps pulling, afraid of Belua's wrath if he slacks.

Tsara and Mitombo, lacking any means to attack the boat from range, wade into the water looking to grasp the floundering sailor and pull him to shore.

Marine 2 takes aim at Tsara as he charges into the breakers and fires another musket ball, adding a fresh plume of smoke to the general haze floating on the water's surface. Attack: 6 Damage: 2

Distracted by his wounds, the marine's aim is poor and the shot splashes into the water harmlessly.

Meanwhile, Thea has taken advantage of a shallow dip in the sand to all but disappear from view.

Seri, then William.

"Stay down!"  Seri surfaces just long enough to hiss at her islander allies.  "These easterners may be cursed in Trondro's eyes, yet their fire sticks may slay us all the same.  Trondo favors those who trim their sails wisely."  (Belatedly, she remembers that these islanders' simple canoes lack sails).

She dives back under, hugging the sandy bottom and gliding northwest, then north to approach the retreating jolly boat from a (relatively) obscured underwater position.  Sharply-spiked bull kelp lashes from her free hand up at the nearest oarsman in the stern, yanking them out of the boat by the throat and drawing them towards her, down into the depths.  Another gust of wind follows, slamming another rower away from her her, northeast and upward.

Movement:  (30' swim speed and underwater breathing) NW-->NW-->NW-->N-->(Action:  Thorn Whip)-->N-->N-->(Bonus Action:  Telekinetic Shove)

Action:  Seri casts Thorn Whip at the nearest oarsman in the port-side stern of the rowboat.  Melee spell attack (30' range):
Thorn Whip to hit:  24
Thorn Whip damage:  6 magical piercing
Forced Movement on hit:  10' towards Seri (SW and down underwater), no saving throw for large-sized or smaller creatures.

Bonus Action:  Seri Telekinetically Shoves a starboard-side rower.  DC14 STR Save or be pushed 5' away from Seri (NE and up, hopefully out of the boat).

For both the Thorn Whip and the Telekinetic Shove, Seri is hoping that the oars leave the boat along with the oarsmen!

William hears Thea's shout to target the oarsmen and changed targets appropriately. No longer did his arrowheads line up with Belua but rather with the people next to him in the boat.

Attack roll to hit the most damaged oarsman: 24
Damage roll: 8

Seri's spiked sea kelp winds itself around the body and limbs of the nearest sailor, whose mouth opens in panic and confusion, only to be filled with seawater as he is dragged over the side and into Trondro's embrace. Tendrils of red mark his passing into the water as the spikes tear at his flesh.

Str save: 18

The sailor on the starboard side grips hard onto the bench and the side of the boat as another sudden gust threatens to lift him from his seat. He hangs on, as his oar catches the water and is flung around parallel to the gunwale.

However, as he does so, William's arrow thuds into his chest, killing him instantly. He slumps backward into the lap of the rower behind him.

There are now two sailors in the water and one dead in the boat, leaving three on the oars.

Billy pushes forward, wading and then swimming out towards the first sailor that was ejected from the boat.

Sailor 6, who had just been released by Seri's Thorn Whip, struggles to the surface, gasping and spitting, and looks around desperately trying to locate the boat. He begins flailing ineffectually towards it.

End of Round 3.

Order for Round 4:

Sailor 1: 20

Marine 1: 20

Herim: 18

Belua: 16

Sailor 2: 14

Sailor 3: 13

Sailor 4: 13

Kairoiani: 12

Interpreter: 11

Thea: 10

Tsara: 9

Mirombo: 9

Marine 2: 9

Seri: 8

William: 6

Billy: 5

Sailor 6: 4

With only three oarsmen, the boat moves a mere 10ft forward.

Sailor 1 continues dutifully pulling at his oar.

Marine 1, shocked by the sudden exit of Sailor 6 entwined by sea kelp, casts about in the water, looking for Seri. Thinking he catches her movement, he fires his musket. Attack: 16 Damage: 9

Herim continues lying prone in the sand, covering his head.

Legati Belua bellows in rage, as his attempts to get to his stricken ship are frustrated yet further. He roughly shoves the wounded Marine towards a rowing bench and commands him to start rowing. He also discharges his pistol in Seri's direction. Attack: 18 Damage: 8

Sailors 2 and 3 continue rowing, whilst Sailor 4, the first to hit the water, starts swimming after the boat, pursued by Tsara, Mitombo, and Billy.

Despite the angle and the obscuring effect of the waves, Belua gets lucky and Seri feels his ball bite into her flesh as she swims.

Kairouani, over to you.

Kairouni moved into the shallow surf and fired again keeping her aim on Belua.....she would rather be killing chiefs than warriors or workers.....she had to stand upright to keep her bowstring out of the water....knowing she made a tempting target.

Move 30' up maintaining Hunters Mark on Belua.

Bow on Belua Attack: 16 Damage: 11 + Hunters Mark 2

Barely has Belua’s shot cleared the end of his pistol barrel than Kairoiani’s arrow arrives like an avenging bolt from the gods. It enters the front of his neck at an angle as it descends from its flight, and the tip bursts from his spine, red and glistening. He is killed instantly, and his limp form slumps into the bottom of the boat. 

His death is met by a stunned silence from the crew of his small craft. The grizzled islander, his erstwhile interpreter, is first to act. Tearing his gaze away from the dead nobleman, he looks towards the ship-to-ship combat taking place out in the bay, and then back towards shore, and the threat that approaches from that direction. Perhaps rueing burnt bridges, he decides quickly that his best chance of survival at this moment lies in surrender. He places down his bow and raises his hands, shouting in accented Verdian, “stop! Parley!” He addresses the unwounded marine, who is busy reloading his musket, “do not shoot any more. We do not have to die here.”

Thea, your move. 

Thea Oltineas

Thea shouts as Kai's arrow kills the leader. She looks at the now trying to surrendering soldiers, and is unsure of what to do. She looks to her companions, wondering what to do? She stands up and beckons forth her friends as she still holds her bow, an arrow knocked, ready to fire at a moments notice.

She looks at her older friends, and asks "What do we do?" Waiting to see how they reply. She guesses they would council mercy, but they've saved her life a few times, so she consults them anyway.

She studies the two men, looking for signs of deception.

Notes

I'm going to ready my action to fire my bow at the marine if he or the interpreter makes any hostile moves that Thea notices. I'll go ahead and roll the bow attack IN CASE we need it.

And I'm going to make an Insight check to anticape hostility before it happens: 22

Shortbow [action]Attack[action] VS. Marine: 11 to hit. Hit: 10 Piercing

DM roll: 8

Thea's assessment of the mood in the boat is that the interpreter does not speak for the majority. The two marines in particular seem in no hurry to surrender, whilst the sailors are looking hopefully towards where the rest of their crew is fighting off the attackers.

Tsara and Mitombo manage to swim out and take hold of the first sailor to have hit the water. Faced by the two islanders, both stronger swimmers than he, he submits to their mercy.

The wounded Marine snarls at the interpreter, "shut up, you runt!" and shoves him roughly aside as he takes to the bench and begins rowing, as Belua had commanded him to do, and as Thea suspected he might. He shouts to the sailors as he does, exhorting them to greater speed to get away from the shore-based threat.

Seeing that the fight has not gone out of the marines or the sailors, even though the interpreter has given up, Thea looses her arrow towards the boat, but it splashes harmlessly into the waves.

Seri, then William please.

Belua's final shot stings Seri's side and her blood trails along the sandy bottom as she continues to glide northward underwater.  Whether the blood is actually blue or just looks blue through the waves is difficult to say.  She grits her teeth and swims until she is directly west of (and well below) the boat.

Once more, she lashes out with her thick strand of spiked kelp at the nearest rowing sailor on the port side, and a gust of wind pushes at another at the starboard.  Aiming to eject both from the vessel, along with their oars if possible, though her focus falters due to her wound.

Movement:  Seri swims north 30', hugging the bottom, now directly 25' west of the boat (and down).

Action:  Seri attacks the nearest (rowing) sailor on the port side with her Thorn Whip (melee spell attack, 30' range):
Thorn Whip to hit:  10
Thorn Whip damage:  1 magical piercing
Thorn Whip forced movement on hit:  10' west and down towards Seri

Bonus Action:  Telekinetic Shove on (rowing) sailor on starboard side of the boat.  DC14 STR Save or be pushed 5' away from Seri, up and east.  Hopefully out of the boat, taking the oars with him ideally.  Trying to send the message that this is just what will keep happening to rowers.

Seri's spiked sea kelp this time narrowly misses its target, a terrified sailor who manages to lean back just beyond its grasp as the tendrils and spikes trail over his clothing. 

Str save: 12

The rearmost sailor on the starboard side of the bench is suddenly lifted from his seat by an unseen but powerful force, and topples over the side, still clutching his wooden oar, which he uses to keep him afloat.

There are now two sailors and a wounded marine rowing the boat. The interpreter has been cowed into silence by the ferocity and determination of the marines, the unwounded of which is standing in the bow reloading his musket. He looks up from his powder flask just in time to see the sailor seemingly leap over the gunwale and into the water, taking his oar with him.

The boat is foundering with only three oarsmen, and is making painfully slow progress forward. Nevertheless the occupants seem to feel that their only course of action is to keep pulling for the dubious safety of their ship, wreathed in gunsmoke as it is.

William, your move.