Guard room fight:

Initiative:
Boko: 13
6 Sahuagin with spears: 1
2 Sahuagin with warhammers: 15
1 unarmed sahuagin with arcane powers: 15

The unarmed sahuagin shouts something in a raspy voice. 
A flash of light streaks toward Boko: 12. Hit: 16 radiant damage.

One of the sahuagin swings its warhammer at the gaseous Boko, twice:
Warhammer Attack #1: 23. Hit: 7 bludgeoning damage. (Halved due to gaseous form.)
Warhammer Attack #2: 14. Hit: 4 bludgeoning damage. (Halved due to gaseous form.)

The other hammer-wielding sahuagin rushes over to the gong and, picking up the metal bar, rings it.

Boko is up next!

(Critical hit damage: +6 but ÷2) 

Meanwhile...

While the shark Oceanus poked only looks slightly wounded, Gerrard seriously injures the second armored beast, while Qivys attempts an arcane bane:

Shark #1 Charisma save: 17
Shark #2 Charisma save: 14

Initiative:
Oceanus: 18 
Gerrard: 14 
Qivys: 13. 
Two Armored Sharks: 2 

The bane fails. The shell-covered sharks attack:

#1 Bite Attack vs. Oceanus: 18. Hit: 21 piercing damage.
#1 Bite Attack vs. Oceanus: 17. Hit: 7 piercing damage.

#2 Bite Attack vs. Gerrard: 24. Hit: 9 piercing damage.
#2 Bite Attack vs. Gerrard: 16. Hit: 12 piercing damage.

Gerrard and Qivys may act!

Gerrard:

Just barely getting caught by the shark’s second attack as his defensive maneuver is less useful than he hoped, Gerrard retaliates with attacks of his own, trying to poke at the shark’s face and disable it.

Elegant Rapier Attack 1:  Attack: 18 Damage: 8

Elegant Rapier Attack 2:  Attack: 12 Damage: 10

Main Gauche:  Attack: 24 Damage: 8

Getting into the flow of the battle, Gerrard prepares to parry if he is struck again.

Qivys will cast Bless on Gerrard & Oceanus (2nd level).

He will then move to the right and hold till he can attack. 

Oceanus again tries to puncture his attacker:

Trident Attack vs. Shark #1: Attack: 24. Hit: 3
Trident Attack vs. Shark #1: Attack: 21. Hit: 7

Gerrard manages to get his blades through the other shark's armor plating, twice. He finishes off one, and turning, sinks his main gauche deep into the other.

Qivys blesses them with an arcane prayer.

The remaining shark, now in a frenzy, turns its attention upon Gerrard:
#1 Bite Attack vs. Gerrard: 7. Hit: 13 piercing damage.
#1 Bite Attack vs. Gerrard: 17. Hit: 22 piercing damage.

Gerrard (blessed) and Qivys may act again!

(Crit damage from Oceanus' deadeye aim: +6.  It's in pretty rough shape.)

Gerrard:

Managing to dodge the first attack, Gerrard throws up a hasty parry to try and slow down the incoming bite, then stabs quickly in response twice, trying to regain his composure.

Reaction to Parry:  9

Rapier Attack 1:  Attack: 23 Damage: 7 plus Bless:  1

Rapier Attack 2:  Attack: 28 Damage: 14  plus Bless:  2

Second Wind:  17

Not having any weapons that will do much damage while under water, Qivys reaches out and punches the shark, casting Shocking Grasp as he does so.

Hit: 13

Damage, if hits: 12

Boko investigation:  8

She looks for cracks with her still-remaining blindsight and darkvision, moving up the outer wall near the door. She doesn’t want to flee out the door, bringing the sahuagin outside right to her hiding friends who are assuredly bored, waiting for her.

If she finds something she goes through it, and if not she goes through the crack in the door.

Gerrard skewers the remaining shark. Combat has ended below.

--

Above, Boko glances around for a gaseous escape. Finding none except a slight gap at the top of the doors, she seems through the ten-foot-tall exterior door into the cold night air. 

The spear-wielding sahuagin push the heavy doors open. 

The arcane wielder again mutters a mystical word and summons a ghostly trident. She steps out into the cold wind and mentally commands it to attack.

Ghostly arcane trident attack vs. Boko: 24. Hit: 10 force damage.

Boko, floating ten feet above the causeway outside the exterior door, may act next.

Boko swirls around the top of the fortress, looking to round a bend or corner to get out of view. Dashing if necessary.

Gerrard:

As the second shark dies, Gerrard looks quickly around to see if his friends are okay or if their fight has been observed (though he has no reason right now to look above water).

Perception:  6

Kei 

Impatience is setting in. "I bet that lot have gone into the ruddy fortress on a fools errend" he grunts as he kicks a crab across the rocky shore. His eyes dart to the sky, keeping a lookout for the enormous bird creature. "I don't fancy doubling back with that creature flying around, guess we just hole up here" after a moment he continues with a seriousness to his tone "or....we could just head for Saltmarsh". 

Boko swirls up into the night air, looking for cracks or crevices to hide in. She finds some – holes and breaks in the weathered, crumbly stone exterior of the jagged island. She drifts inside, out of sight and out of reach. But not before the ghostly trident blindly swings once more at her:

Blind ghostly arcane trident attack vs. Boko:5. Hit: 8 force damage.

Gerrard detects nothing in the shallow murky waters underneath the shore end of the causeway – no sign of additional trouble, just two dead Shell Sharks drifting in the sluggish currents.

Both instances of combat are over (at least for now.) Boko remains in hiding. The whereabouts of the sahuagin who pursued her out the causeway door is unknown.

Meanwhile, Kei and Peri monitor the seas and skies for trouble on the beach, but see nothing. They do, however, note the subtle lightening of the skies to the east. Dawn – or Saltmarsh's dull, disappointing version of it – may finally be near. 

Boko dodges again. She descends the fortress wall, out of sight of the front door, all the way to the surface, and then moves around the other side of the fortress along the water. The spell makes the water almost a solid surface to her so she flow-walks along the waves creating and ebbing, making her way to where she left the party. Assuming no interference, she shifts back out of gaseous form, and a giant spider tries to grab on to the shore side of the causeway pylon. Whether or not she succeeds, she shifts back to herself and drops underwater, looking around for the others. “Do you want the story right now, or on our way back to Kei and Peri? Tom was not there.” She is ready to head back to Saltmarsh.

Gerrard:

Breathing a sigh of relief, Gerrard smiles when he sees Boko, but he can't hide his disappointment when she returns alone.  He says, "It has been a long night, and dawn can't be far off.  If Tom is beyond saving, let's get out of here and meet up with the others before our ride leaves.  You can tell us the details on the way back once we are above the water and on the ship.  Shall we get moving?  What is our best path to remain undetected?"