[placement at the start of combat was arbitrary, so I will say Eron could have been close enough to Tilliman to attempt his attack on the monster.  It fails to hit.]

As Tilliman tries to swim away, the demon ray contorts its body in the water, bringing its long, barbed tail to bear and attempting to take the hooked spines against its fleeing prey.

Barbed Tail AOO:  Attack: 20 Damage: 9

(editing to get rid of ‘dice rolls not available… I am on the road this evening otherwise I would have rolled in the encounters tab and just posted the results.)

On his turn, Therris unslings the crossbow from his back, nocks a bolt, and pulls it into place. He takes careful aim, bracing against the edge of the boat, and lets a bolt fly after the ray pursuing the shouting Tilliman.

Seasick Con Save: 18

Feeling a bit unsure of herself with the whiskey and the waves, Fane will shoot her Light Crossbow at the lone ray towards the front of the boat.

Attack disadvantage: 7 (natural 1 + 6), damage 9

Regdus sees the creature coming full speed towards him and realises too late he is a prey. With a quick look behind his shoulder, he estimate the distance between him and the boat. If not in melee, Regdus will try to swim back toward the surface and his companion.

The stress of the battle makes the crystal lodge in his chest crackles with energy and with a loud bang his trows a lightning ball at the creature.
Attack: 11 , Lightning damage :4 

[Regdus is on the surface, and the Ixitxachitl is within melee.  Does he still swim away?]

Quote from Bartjeebus >>

[Regdus is on the surface, and the Ixitxachitl is within melee.  Does he still swim away?]

 Then no, staying within range

Eron swims toward the demon ray pursuing Tilliman and tries to fight it off. 

As Tilliman makes his escape, the Ixitxachitl's barbed tail completely misses him.  With a burst of strength born from fear, he hauls himself up over the side of the rowboat, collapsing prone at the bottom, soaking wet.

Fane fires a bolt at the Ixitxachitl off the prow of the boat it doesn't come close.  [For ranged attacks coming from above the surface, anything under the surface has 1/2 cover.]

Regdus' panicked spell attack misses the Ixitxachitl, but the crackle of lightning in the water seems to unnerve the aberration.  

Regdus and Eron both hear a gurgling, choking sound coming from the three demon rays.  It seems as though they are communicating with one another, but it is not a language either of you can understand.

The Ixitxachitl menacing Regdus chooses to disengage from melee with him, carefully swimming around him and making around the stern of the boat, toward Eron.

Seeing that Tilliman has escaped, the Ixitxachitl hunting him turns to Eron instead.  It turns its wide body sideways to try to deliver a bite from its fanged maw.  It bites down deep on the arm that had tried to punch it, speading blood in a cloud around Eron.

Bite Attack:  15  Piercing Damage:  6

The Ixitxachitl off the prow of the boat also turns away from Regdus and toward Eron.  It swims to within range and then also makes an attempt to bite.

Bite Attack:  10  (Miss)

[Eron still gets a turn this round...]

Therris carefully takes aim and, accounting for the refraction in the water, lands a bolt in the rubbery flesh of the Ixitxachitl that had menaced Tilliman and then bit Eron.  It looks like a solid hit... the creature is still alive but there is black blood spreading in the water.

Round 2

1. Fane
2. Tilliman
3. Regdus
4. Ixitxachitls
5. Eron  <---- still has a turn this round
6. Therris

Round 3

1. Fane (drunk but no longer seasick)
2. Tilliman
3. Regdus
4. Ixitxachitls
5. Eron
6. Therris

[Also, everyone would be aware that, as noted in post #79, Eron is tied via a length of rope to the gunwale.]

Eron's arm bursts with pain although after the recent past the pain is not as distracting as the blood spreading in the water, then as if by some miracle a couple of bolts hit the water one striking the very creature which has bitten him.  He tries again to land a blow but with an 8 I'll assume there is no strength behind the swing... seeing the other small rays converging on him Eron does begin to feel a sense of dread, 'at least Tillman made it, gotta buy them some time to escape' are his thoughts...

Tilliman rises from being prone, looking over the edge of the boat at the creatures doubling up on Eron. He grits his teeth, reluctant to risk losing one of his good daggers, but more reluctant to watch one of his new companions die in front of his eyes.. He draws one of his daggers out and launches it at the nearest creature perusing Eron, shouting, "Get! You bottom-feeding mud-slug! Swim back to whatever hole you came from and leave him alone!"

 The gnome curses as his dagger disappears into the water...

On his turn, Therris will fire upon the least injured demon ray. If one ends up making it within 5 feet of him, he'll instead drop his crossbow into the boat as a free action, draw his sword as an object interaction, and thrust it into the back of the elasmobranch. 

Fane drops into a familiar spell and casts Shape Water, Instantly the water around Eron turns opaque, refusing to take in light.

OOC: I’m hoping that turning the water around him is like cloaking him in darkness, at least under water, offering him an advantage to his AC or disadvantages to the rays trying to hit him!

Regdus sees the 3 rays and wonders why it did flee from him. Maybe he is a bit intimidating. He decides then to swim toward the stern of the boat and attack the creature with his dagger, while creating some electric energy, then moving away - back to the boat if possible.

Attack: 21 for 5 or piercing damage + 2 thunder damage if the creature moves

Fane gets a clever idea and, with a cantrip, renders the water around Eron completely opaque.  To those observing, he appears to suddenly be encased in something like a dark ice cube, though from his head bobbing above the surface, it is clearly still fluid water.  

Tilliman throws a dagger at the offending rays, but it plunks harmlessly into the drink.  Eron makes a grab for it reflexively, but he loses sight if it in the very opaque water that hides him, and it sinks to the bottom. 

Regdus swims in pursuit of the Ixitxachitl near the stern, stabbing it violently with his dagged and cutting a wound in its fin.

The ray near Regdus [action]disengage;disengages[action] again, but when it moves away toward Eron, there is a roar underwater as booming blade is triggered.  The creature startles and swims away, gurgling something in their strange language.  

The two Ixitxachitl's near Eron circle in frustration, unable so see him clearly in the magically obscured water.  When Therris fires another bolt at them, even with a miss, they decide these pickings were not as easy as they had first believed.  

The three Ixitxachitls all swim off, disappearing into the larger school of manta rays.

[We are out of initiative.]

Eron and Regdus are able to rejoin the others inside the boat, if they are so inclined.

Eron jerks in fear as the water colors around him, thinking it’s the demon rays doing, but as another bolt flies and the creatures swim off, he breathes in relief.  Before boarding the boat Eron makes a quick dive to possibly retrieve Tilliman’s dagger.

 Either way he then climbs back in, “I can’t thank you all enough.  I thought I was about to get ripped apart.  Perhaps we should report the presence of these creatures, unless you’d all like to try the deep area.  I won’t protest either way at this point.  And thank you Geralt you’re piloting is exceptional!”

Tilliman lays back agains the boat, catching his breath... "That..." He says. "...what not what I was expecting when you asked us to come out there."

With a quick dive to the bottom [it is still shallow here atop the reef] Eron is able to locate Tilliman's dagger, resting gently atop a piece of pink coral.  

Geralt grumbles an acceptance of the compliment, but then asks, "Have you truly not found anything yet?"  He casts another anxious look to the south.  The clouds continue to grow and assemble into larger and more unsettling formations, though the rain still looks more than an hour off.  That doesn't mean the winds don't continue to increase, worsening conditions on the surface of the sea.

“Let’s check the deep water before we call it quits, see if we can get something out of this search?” Fane offers.

Therris slings the crossbow onto his back, and sits down in the back of the boat. "Damn rays. They get me sore as a skipper's thumb, never had to deal with any up close like that before, but I know firsthand they'll make a fistful of kippers outta ya if you're not careful. Best to head in before the heavens get to hateful, rain's coming on and I've managed to stay dry enough thus far. Maybe a quick look-see 'round deep waters, but no more'n half an hour, and preferably half that. Wouldn't do to make acquaintances with the barnacles on the rocks, I hear they've sharp tongues."

Eron does his best to dress his wound as they make their way to the next location.  The fact that otherworldly creatures are in proximity to the shipwreck does not bode well… Eron, “Take heart friends, our efforts were not in vain those creatures were not natural animals and their presence here may reveal a greater threat.  However, this last venture, be cautious and prepared to flee if necessary.  Perhaps we will recover clues or bodies perhaps not but I thank you again for helping me recon this disaster.”