Episode 2

As The Keshet pivots from the angle of the impact, its stern scrapes bangs and drags along the port bow of the Korvaran longship. The oil splash fire still burns along the hull wall of the longship port bow and part of the deck and railing of The Keshet.

There seems to be a commotion up on the railing of the enemy vessel. A few of the raiders who looked like they were about to leap down and board The Keshet look back towards something on their own ship. Cautious and hesitant looks prevail on the faces of most raiders. Only a few throw javelins now. Some step away. None move over the rails to continue the forced board.

Noticeably, Valates' heavy commanding voice has fallen silent.

Heshbaan Raider Javelin Throwers

After throwing, a couple of the raiders move back away from the rails. Those who stay are cautious and taking what cover is available as they reach for more javelins.

Two big myrmidon mercenaries appear up at the rail. In their bronze helmets, the set their shields and raise their heavy war spears. One of them betrays an uncertainty of their stance as he glances back towards the deck of his own boat.

Bolstered by the two soldiers, the less-hardened skirmishers reset on the railing.

On boarding the Korvaran longship
Clarifying height difference, The Keshet to the Korvaran:
I think I did a bad job on some word choice on the height difference. The 6' - 7' difference is from Keshet deck (floor) to Korvaran railing. The Korvaran deck is obviously lower. You guys cannot see the Korvaran deck. But the difference deck Keshet to deck Korvaran is not so pronounced, although that is not really what matters at this point, because the obstacle to get over is in fact the Korvaran railing.

The good news:
A 6' wall is not impassable. Think of it like an obstacle course wall. You would have to jump up, grab it and quickly climb over.
One bonus is that you could use the railing of the Keshet to push up from on the leap.
As the Keshet gets turned and shifted around, the railing of the Korvaran is slanting downward as it goes towards midship.
So the difficulty of this isn't exceptionally high on its own.
But yes, we are talking about an eventual Athletics check.

The bad news:
There is fire along the Korvaran hull, which makes part of the path that you would like to climb a wall of fire. Will it kill you? No. It's a limited fire from an oil splash. Will it sting? Yes probably. If you are exceptionally bad on the boarding attempt, you might misjudge and stumble, bringing yourself too close to the fire. There could be damage there, if you really botch the roll.
Yes there are defenders on the rail. Most have stepped away for some reason, but still four are standing ground:
2 Myrmidon and 2 JT Raiders
These guys are at half cover.
These guys have shields, so they may have options of defending against missile fire.
And to be fully clear: if someone is going up and climbing the rail, Korvaran defenders will likely have Advantage on Attacks against Keshet climbers unless there is some change of circumstance.
The window of opportunity for making the transition ship to ship may be closing. As mentioned, the Keshet is getting spun around and gradually pushed away since the boarding and grappling effort went so poorly. The Korvaran is gradually driving by. In time the ships will separate. Exact timing will not be revealed, but predict that the next 3-4 rounds will be important to complete the shift, as Difficulty of the Athletics task to make the jump and climb will get more severe over the longer timeframe as distance of separation will increase.

Commitment to realism
This is a very possible task, but a challenging one that has danger and risks that must be faced. And this may be outside of normal give and take on Initiative order, as these opponents my set Ready Actions with triggers. And in any case, there will be a realistic nature of the fact that climbers have some time investment for climbing while defenders might be able to time their attacks to exploit that. So ahead of time, understand there is no real way to mechanically game the combat sequence to complete Movement and reset somehow to completely nullify all of the advantage that higher ground defenders do have in a practical sense.
Also, you have to address the real act of climbing which means having both hands free. Accommodate for weapon stow and draw management. Or is it best to toss cumbersome weapons up and over and then go after them? I don't know, but figure out something that is real and we will incorporate it into the visuals and narrative timing of the whole thing.
Also, mentally address gear that you would realistically have with you. We don't know yet if you are leaving everything on the ship behind forever. The Keshet is not sinking in the next 30 seconds. But you may be leaving things behind for now. Or maybe you are going to grab things now and make the time investment for that. Up to you, but plan it out accordingly with an eye for realism.

So... next entry will be a summary of mechanics.

Round 3 Boarding Mechanics

Success: DC 12 STR Athletics

Failure risks: not making it up and falling back to Keshet deck, unintentional shift into range of oil fire, falling between ships into river

Requirements: must have 2 hands free

Limitations on spacing: only 2 people may attempt boarding in Round 3

Defenders: 2 Myrmidons, 2 Raiders, armed with shields, spear, javelins, sidearm blades, 1/2 cover to range fire

Myrmidon Battle Maneuver Save (from Ivor)

Ivor viciously throws his trident. The weapon crashes into the soldiers shield, knocking it back, and continues on to ricochet off of his bronze helmet. Stunned by the blow, the mercenary trips up and falls over backwards.

Attack, Trident Throw ranged against Myrmidon: Hit. Battle Maneuver successful.With the small gap in the line, Ivor leaps from the railing of the Keshet and grabs hold of the higher railing of the Korvaran. As he tries to pull up, the other mercenary slams into him, trying to stab and shove him from the side. One of the raiders hurls a javelin at Ivor at extreme close range.

Myrmidon Attack: War Spear
Bonus Attack: Shield Push

Result: Critical Hit on Spear Attack. Total 15 damage.
Required Action: Ivor must succeed on comparison of 16 on STR Athletics or be Shoved off of railing.

Suddenly a huge volume of fog swirls up into the space between the two boats. The entire area at the back of the Keshet and the front of the Korvaran is covered in thick opaque fog, effectively blinding those within and hiding them from those without.

Ivor, Maeriks, Kray, Thorn suffer Blinded condition.
For Bryn, the stern of the Keshet and the front 1/3 of the Korvaran is Heavily Obscured.

Was imagining sheer brute force of the throw entering his skull would knock him over. But okay if we have to change it then I guess Menacing is the only real candidate.

Ya, that happened. I went with same idea.

Resist Shove attempt.

As the fog billows in, Thorn hurries forward before he loses sight of the railing. As the boat continues to pivot, it becomes difficult to maintain sense of direction and distance as the fog thickens rapidly. Pressing on through the risk, Thorn is able to grab the top edge just before his vision is completely overcome.

As Thorn grips the rails a raider steps in to intercept him. The Heshbaan sets his shield against Thorn's chest and shoulders and tries to shove him loose.

Required Action: Thorn STR Athletics to oppose Shove Attempt. Must match or beat 20. Note: no Disadvantage or auto-fail to this check. Sight is not required in this strength and leverage contest.