I'll give you the wording from the Darkmantle's Darkness Aura ability...

Darkvision can't penetrate this darkness, and no natural light can illuminate it. If any of the darkness overlaps with an area of light created by a spell of 2nd level or lower, the spell creating the light is dispelled.

Hence, both the light from Rhayna's Light cantrip and Guiding Bolt spells were both dispelled, therefore no advantage from Guiding Bolt. You can cast it again for the damage, but the light won't work.

(What about light from Dawnbringer? Sekkar is trying to bring second boat closer to the first one, can his sword dissipate darkness?)

The light from Dawnbringer is sunlight and is therefore natural light, and can't illuminate this darkness unfortunately.

OK, so on second thoughts, Dawnbringer CAN illuminate this darkness. I think it's fair to say the light is magical sunlight, and not natural sunlight, and since it isn't created by a spell of second level or lower (or any spell for that matter), then it is also not dispelled.

 Kaylyn steadies herself in the boat.  She had felt the creature was pulling her into the water but she was only losing her balance instead and she was able to regain control. 

She looks around. 

"Where did the other boat go? Is it in that darkness or has it disappeared?"

(Not sure whose turn it is.)

Sekkar will spend his round if needs to be to catch up with the first boat and illuminate as much as he can with the sword.

Rhyana is surrounded by darkness and she can't see a thing. She has no choice but to try casting Guiding Bolt on the creature attacking her again.

Attack: 9 Damage: 23

(Sorry, forgot to roll with disavantage, but it missed anyway.)

So for Round 3... actions higher in the initiative are changing the situation, so let's summarise.

Kaylyn and Sekkar have the same initiative, and can therefore choose who goes first. Sekkar is moving the boat and illuminating the darkness with Dawnbringer, so if Kaylyn wants to go second, then she can take an action with the full benefit of vision from bright light.

All other characters can also therefore act in bright light. As things currently stand, Rhayna is casting Guiding Bolt (which will probably miss anyway, unless something gives her advantage), Kastix is casting Fire Bolt (which will probably hit).

Zeno probably doesn't have a reason to jump out of the boat now, as the two boats will be adjacent by the time he takes his turn, so can take an action.

Need an action from Joetaro also, who is in melee range of a darkmantle, but not attached to it. He is under the benefits of Sanctuary, which will end if he makes an attack, casts a spell that affects an enemy, or deals damage to another creature, but any spell or action that benefits another player won't affect the Sanctuary.

Sarith is trying to detach a darkmantle from Rhayna, and Hemeth is fighting against one attached to his head.

Joetaro are going to attack the darkmantle. 

Attack: 20 Damage: 8

Kaylyn readies herself as Sekkar's blade begins to illuminate the area.   Readying her magic she will to blast one the creatures with her Eldritch Blast when it comes into view. 

(Sekkar goes first)

Attack: 28 Damage: 11

ROUND 3

I sense darkness! Take me there! calls the voice of Dawnbringer in Sekkar's mind. Instinctively, Sekkar lays his sword across his lap and reaches for the paddle, glad to notice that the blade of sunlight does him no harm. Expertly he turns the boat and propels it toward the zone of darkness, dropping the paddle and holding up the sword as the vessel glides forward. As the light from Dawnbringer penetrates the darkness, the sword's voice rings in song out across the water to be heard by all in an exultant hymn:

Gloom and Twilight, Dusk and Shade
May you flee before my blade!

The aura of darkness is pushed aside like mist before the wind, and the scene in the other boat is revealed to all. Sarith pulls at a darkmantle attached to Rhayna's shoulder, Joetaro glows with a soft blue nimbus as another darkmantle hovers just above and between the two boats, and Kastix sits beside Hemeth who has a darkmantle covering his head.

Immediately Kaylyn lets loose a bolt of arcane force which strikes the creature attached to Rhayna. It releases its grip on the cleric, and Sarith tosses the corpse into the water. Joetaro speaks a divine word of power and then slashes out at the hovering darkmantle, causing the blade to flare with heat as the sword strikes and setting the creature on fire. (Dawnbringer gives a small ooh! of approval.) Rhayna shoots another mote of moonlight at the flaming creature but it flies wide.

Zeno draws his rapier, stands in the boat, tenses, and then leaps across the gap between the two vessels, stabbing the flaming, hovering darkmantle with his sword. As he lands in the second boat he flings the dead charred corpse far into the lake. There are now six bodies in the boat and it is rocking wildly, but Kastix flings another mote of fire at the creature on Hemeth, and it too falls into the water with a loud sizzle.

There is a moment where no-one speaks and the only sound is the lapping of the ripples against the boats as they slowly stop rocking, Hemeth's laboured panting, and a hiss of steam from the two once-flaming corpses in the water. Within moments though, the water begins to churn and the corpses are surrounded by dozens of small fish that begin to feast on the darkmantle flesh.

"Satisfied?" Sekkar was no less pleased than his sword and allowed both of them to enjoy the moment before he added: "Be careful when you come out - we do not want to scare the darkness away before we are ready to attack it, right?" Feeling as he gave enough incentive for the time being he added out loud: "Gone. Good. I say we move away from that spot." And he rowed the boat to position it again behind the main one.

Once Rhayna is freed from the darkness, she sees that it was Sarith that came from the other boat to help her. "Ohh. Thank you Sarith. It was very brave of you to come and help me." She checks around to see if anybody needs healing.

(Does anybody needs healing?)

Joetaro shakes his head. "I don't think I want to go fishing any time soon." 

Placing a hand on himself while casting Cure Wounds on himself, "I can't wait until we get out of these lands,"

Healing: 11

Kaylyn relaxes as the last creature falls into the water.  She reaches up and scratches Pensive behind the ears absently.

"Let's get moving again shall we?  I'd really like to get off this lake."

Dawnbringer acknowledges Sekkar's directions, and seems happy for her wielder to take charge and give directions, promising not to 'come out' until he wills it. After some healing by Rhayna and Kastix, the journey continues. All the travellers are still very cautious, but no more tentacles drop from the ceiling and there are no more warnings from Pensive, although Joetaro does notice several more schools of the carnivorous fish. The group journey for the rest of day, then stop to forage and sleep, before continuing again. The low ceilings with stalactites persist for another half a day, then the scenery dramatically changes to a more open area with multiple fast-flowing rivers and faerzress pervading the area. Paddling is more difficult, and by the end of the day's efforts, all are exhausted and looking forward to rest.

But rest that night comes with difficulty, and under the dim light of the faerzress, all slumber fitfully through vivid dreams.

Rhayna:

 Kastix:

 Zeno:

 Joetaro:

 Kaylyn:

Sekkar:

The seventeeth day of travel features more upstream rivers with lots of strong currents and rapids to navigate, but the group is used to it by now and Rhayna continues to call on knowledge from Selune to help guide the boat. On the next day, signs of civilisation start to appear, and the group avoid several waterfalls by passing through locks similar to those early in the voyage. The occassional duergar can be seen navigating their own boats through the lock systems.

At the sight of these other duergar, Hemeth finally starts to speak more candidly: I've decided - I'd like to show ye the way to th' surface that I use. Ye saved me life out 'ere, more'n once, and it's the least I can do. But before that I want to fix me 'and, (and he holds up his stump) and see if I can 'ave someone fashion me a replacement o' sorts. It might take a few days though, maybe a tenday, so ye'll 'ave to be patient - and careful! I've learned to trust ye, but the other duergar won't. Some'll probably prefer to enslave ye as to do business with ye. Ye'll get better 'elp if you try to gain their trust as you 'ave mine.

More duergar in boats are seen on the nineteenth day. They seem to be an extremely suspicious bunch, and despite Hemeth's presence, most of them glare curiously and cautiously at the two passing vessels. Again Hemeth voices some helpful information: When we get to the city, ye'll 'ave to go to Ghohlbrorn’s Lair. It's the only inn that outsiders are allowed at. I'll point ye the way. Ye'll not find better arms and armour anywhere in the Underdark than in Gracklstugh, but just be wary o' who ye talk to in the Blade Bazaar.

On the final day, after just a few hours of travel, the first faint sounds of clanging can be heard drifting through the caverns. It grows in volume minute by minute until it seems to echo and reverberate off the cavern walls, and is soon accompanied by a faint red glow and an acrid taste in the air. Rounding the final bend in the river, the waterway opens into a wide, open cove filled with flat-bottomed zurkwood boats and rough stone docks.

Abutting the shores of the lake, and extending back as far as can be seen, is an enormous cavern. Spires of rock pierce the void like giant columns randomly spread through an immense hall, but the spires are penetrated by windows and archways of all sizes, are linked by chunky stone walkways, and at their lower extremities they appear to merge into more formed and regular structures. These buildings are austere and bland, and have stark square angles and featureless walls. The city looks at first like it is on fire, as an angry red glare and a smoky haze permeates the entire area, but the constant clanging and the purposeful movement of the increasingly-visible inhabitants, reveal that this is the normal state of affairs in a city saturated with smelters, refineries, forges and all manner of industry.

The two boats pull up to a dock where a stocky armed duergar saunters over and inspects the arrivals with a squint. Then, speaking to Hemeth in a language only Sekkar can pick up snatches of, the guard converses with Hemeth:

"Hail, bhràthair! Dè an gnìomhachas?"
"Neach-malairt Hemeth a ’dol còmhla ri daoine os cionn na talmhainn, thig airson ceannach."
"Tha iad a ’tuigsinn na laghan, ceannaiche Hemeth?"
"Tha, bràthair, agus air aontachadh riutha."
"Glè mhath, lean ort."

After this exchange the guard leaves and Hemeth assists everyone out of the boats. He awkwardly counts a few coins to a dockhand, then leads the way heading away from the docks. To the right, a busy marketplace can be heard, and the din of people arguing nearly drowns out the hammering coming from the city forges. A plain stone staircase descends below ground level, and Hemeth indicates for everyone to enter. At the bottom of the stairs two heavy zurkwood doors open into a dark, cold and cramped chamber. Several tables and chairs are scattered around the rom, and several small twisting halls branch off. This is where I'll leave ye says Hemeth gruffly. I'll be back in a few days to check on ye and let ye know 'ow things are looking fer yer departure. If ye 'aven't any questions, I'll be on me way.

"We should probably sell what can while keep an eye out for what we need while moving as a group," Kastix whispers, "They're not much better than Drow in surface people, they're just not hunting us,"

"Thank you Hemeth, for the safe travel and guidance.  You have been very helpful."  Kaylyn says after she gets out of the boat and stretches. "Are there any shops you recommend for armor and goods?  Any contacts that would be friendly to us as outsiders that we could mention your name?  Which way to Ghohlbrorn’s Lair?"

After hearing his response she turns to the others.

"Let us find this inn and settle in.  We will need supplies if we continue on foot."

((If I am not mistaken, we are already in the Ghohlbrorn’s Lair and the market place is just a staircase away, looks like Hemeth brought us there himself))

The dreams were getting annoying, though the lizardman still did not see reasons to panic (cold blood can be useful) - no wonder he dreams about endless tunnels here.

At the docs (he was both - glad to see the land and very alert to the crowd) Sekkar shook Stool to share some spores and translated duegars exchange telepathically underlining "the law" part - better to behave. "You can say that again," Sekkar nodded to Kastix, "and add "yet" to the hunting part. The owner might know who can we sell goods to somewhat fair."

“Thank you Mr. Hemeth for fulfilling your promise. We are the ones that own you a great debt. I do have a question... Do you happen to have a library here, or even a bookstore would suffice.” Rhayna puts her hands together, almost like she is praying, and squints her one of her eye hopeful for a positive Answer.