Nimbly dodging the gargantuan, grasping claw, Gerrard takes advantage of the situation to plunge another blade deep into a soft spot in the crab. It's not doing well.

Oceanus swims forward again and attempts to plunge his trident clear through the massive beast with both hands:
Trident attack: 18. Piercing damage: 4

 He gouges another hole in the mighty crab, but still it keeps coming, squeezing Peri ever tighter.

Everyone may go, starting with Peri. (Escape DC 15.)

Peri:

The huge claw crushes him, nearly causing a gasp that would lose precious air. He feels his bones grind together and something inside him makes a nauseating pop. He thrusts his spear into the crab's claw (18 for 10 damage) hoping it will release him. When it doesn't he begins to panic, thrashing against its grip, but it feels too strong for him to escape. Then he focuses his Ki and rains down a Flurry of Blows onto the claw (15 for 4 damage, and 22 for 4 damage) hoping to kill the beast or at least disable its claw.

Whether it was the spear or the blows is unclear, but Peri's panicked frenzy somehow manages to detach the claw entirely. It doesn't come off cleanly, either – it tears away a large part of the front of the crabs shell with it. The water clouds further with more crab blood, and the gargantuan crab becomes still. Combat is over.

The claw, however, does not release Peri, who is still trapped and losing air fast. (DC 15 Strength check to pry it open.) 

Gerrard:

Swimming down quickly to Peri, Gerrard braces his hands and feet on the clenched claw, then uses all of his strength to try and pull it open enough for Peri to wriggle out.

Strength:  7

(+3 instead of +1 if you allow Athletics to apply, or a help action to Peri’s Acrobatics to wriggle our if you like...)

Dwal

Seeing the distress of the group and Peri (I assume) Dwal will swim down:
Athletics: 18

And will attack the creature. Dwal will cast Guiding Bolt at 2nd level.

Hit: 22
Damage: 22

And misses like a blind man!!! blast it!

Peri:

He'd been certain that killing the creature would release him; finding himself still trapped in the claw's death grip Peri contorts himself to slip out of the claw. 9 +5 if Acrobatics applies; -1 if it's a straight Strength check.

As he wriggles free of the claw he makes a beeline for the boat. He surfaces with a gasp, then pulls himself aboard groaning. He lays there panting for a moment, then turns to look at Kei. "Well, I guess we know what made did in the Sinker. Where's the dwarf? Don't tell me he jumped in to save me? Always gotta be the hero, that one..."

Gerrard:

Convinced he helped Peri escape the massive claw, Gerrard makes sure the halfling safely reaches the surface.  Taking a deep breath himself, he goes back down with his glowing rapier to check on the status below—the crabs, the body in the net, and that nice fishing pole.

Nearly out of oxygen, with a superhuman (superhalfling?) burst of adrenalin, Peri finally manages to pull himself free of the massive claw. He surfaces gasping. The sun is setting, and the cold winds continue to gust heavily. 

Below, the (ordinary) crabs begin to disperse, and thin out somewhat around the massive dead crab and the wreck. The corpse of the sailor, still wrapped in the net, drifts nearby waiting to be hauled in.

Oceanus nudges Dwal and points into the distance, where Dwal can barely make out a glimpse of a human boy, maybe 9 or 10, with pale skin and long, stringy, waist-length hair swirling around him. He is missing both legs and one arm. A look of alarm on his face, he rapidly and efficiently pulls himself along the deep seabed with his one arm, away from the party and toward deeper water. Dwal sees him for less than a second, and then he's gone – vanished into the murky water.

Gerrard:

Diving down and retrieving the fishing pole now that the crabs are dispersing (roll needed?), Gerrard swims over to the body in the net and tugs three times on the attached rope.  “Thank goodness for his glowing rapier” he thinks as he traverses the murky water.

Kei

One. Two. Three. That was the signal. Gerrard must've secured the body. Kei begins his hand over hand technique. Tried and true from his days on the open seas. His grip falters very briefly as a small top-knotted head breaks the surface of the water of to his left. "By Masks many faces man! Get in the boat." He pauses mid tug and offers a hand to the halfling. Once Peri is in the boat he unties the clasp on his robe and passed it too his friend. "Here, get warm. Just don't pick at any of the stitching inside" He resumes his retrieval of the netted corpse.

Dwal

Dwal tries to dive a bit deeper to retrieve the corpse of the boy. If he fails he will return to the surface. Whatever the outcome, once Dwal breaks the surface he will exclaim; "Dwarves and Water don't mix!" 

Dwal will return to the skiff and do his best to take up his position to row when needed. 

Athletics: 6

Peri:

Once he rests for a few minutes he assesses his injuries: nothing permanently broken, but he's going to be feeling it for a while. He knows he can't lay on the boat for too long or his bruised muscles will stiffen up, so he dives back over the side (Acrobatics 10 for panache) and swims down to the Sinker to search for the other body (Perception 22). If he can't find the body on this dive he will assume it was eaten by the giant crab.

Dwal

Dwal will help Peri into the boat: "Rest a moment lad, that beastie almost sent ye to the next world." he says.

With that Dwal will cast [spell]Cure Wounds[/spell at 2nd level on Peri.

Cure Wounds: 7

Dwal swims in the direction of the boy – it was no corpse, exactly; while pale and mostly limbless, what he glimpsed was quite alive – but finds no trace of him. He clambers clumsily back into the boat.

Peri, who really ought to be resting after his nearly fatal crushing, dives back in. He finds no signs of any other bodies inside or outside of the sunken ship. He returns to the boat for some welcome healing by Dwal. Everyone is bitter cold and beginning to shiver.

Unfortunately, there's no room in the little skiff for three live sailors and one dead one. Somebody besides Gerrard and Oceanus needs to swim back. The cold is beginning to seep into Gerrard's bones as well, even as his armor provides a modicum of resistance. Only Oceanus seems unaffected by the temperature. But it's time everyone gets back to shore and in seek of warmth and shelter.

Peri will swim back to shore as planned. Cold and beat-up, he will not attempt to Dash but swim at his normal pace (35ft/2 rounds, so about 3 minutes to get back to shore?)

Gerrard:

After retrieving the fishing pole and making sure the fisherman's body was secured, Gerrard will swim back to shore with Peri, inviting him to relax and using the power of the armor to propel them both at a reasonable speed back to shore.  Struggling onto dry land, Gerrard quickly goes to the cottage and strips, trying to heat up by the fire as quickly as possible before any possible frostbite sets in.

The party reaches shore in the growing darkness of twilight, most on the brink of exhaustion from exposure to the biting cold in their drenched condition. Lamps have been lit in town; bats flit about catching unseen insects. The young acolyte from the temple greets you and helps you haul the skiff up the beach. He'll fetch a wagon and another acolyte to take care of the dead sailor. Your hired carpenters have left your cottages for the night.

The cottages are still about two days away from full habitation. You've been using it this afternoon as a base of operations for the Sinker mission, but the cottages are not yet finished. One of them still has a whole section open to the elements, and there are tools and materials everywhere. One of them has a functional hearth, but no wood to burn. You could camp out here tonight if you wanted; but your current lodgings will be much more comfortable. 

As you recall, you have a 100-year lease on two adjacent cottages at Crabbers Cove. There are four cabins in total between the Carpenters Guildhall and the end of the road and beach to the east. The first (westernmost) cabin is almost completely demolished; the last (easternmost) cabin is dark and ominous and occupied by a family of rats (and recently, crabs as well.) Your two cabins are in-between. They have two bedrooms, a hearth/kitchen and a common room each, and the builders have erected a neat stone wall around both cabins. Both have substantial root cellars (accessible via trap doors) underneath - the stones of which look quite ancient.

The cabins will be ready to occupy full-time soon; they'll have a great view of the ocean, although somewhat exposed to big storms.

Gerrard:

Seeing the cottages cold and dark, Gerrard walks with Peri and the fishing pole to their rooms at the inn.  “Master Fenwader, I will see you down by the fire,” he says as they part.  Changing into dry clothes and coming down to sit in front of the fire, he waits for the others to come join him while looking curiously at the fishing pole, thankful to be young and healthy once more.

Kei

As the skiff reaches the shore Kei helps the acolyte unload the netted corpse. He looks around to try and find Gerrard and Peri but fails to find them. "Let's hope they've made it to the inn already" He turns to Dwal "Let's go Captain" he says with a smirk "you've rowed your last row for the night. First round of spiced mead is on me". He gathers his belongings from the cabins and heads into the town proper.

Upon reaching the Goat he orders a round of warm spiced mead and whatever stew Lankus has cooked up for the group. Gerrard is sitting by the hearth examine the silver pole. "What a remarkable piece of equipment." He says on approaching the noble. "I have drinks and food coming. Hot stew to help warm you swimmers". 

Gerrard:

Suppressing a shiver, Gerrard responds, “My thanks, Master Kei.  You are welcome to evaluate the fishing rod yourself.  While it is a fine example of its type, I have no skill in identifying any magical or unusual properties.”