Seri Arcana plus Guidance:  13 + 1 = 14

Kairouani Gathered up the rope, untying it from the tree, and attempted to cross the river to assist.

Acrobatics- 10

William, having grabbed hold of a sturdy vine to brace himself against the current, tried his best not to get battered by the heavy flow and keep his head above the water. He spotted Thea approaching the river up above him.

'Help!' He shouted. 'Get me a rope or something!'

Is a check needed to keep holding on?

Whether or not she thinks she can grab the metallic object with her invisible Mage Hand, Seri won't do so yet.  As soon as Kairouani crosses over successfully, Seri leads the Igneri woman to where Thea is, at the ravine's northern edge above William.

Seri then mumbles to beseech Trondro's Guidance once more, hurriedly tying her rope and Thea or Kairouiani's rope together to create a longer rope:

(Not sure if a check is needed to tie ropes together securely.  Seri's Survival plus Guidance:  8 + 2 = 10 or 15 + 2 = 17 with help).

She then lowers one end the rope to William, tying the upper end around a tree.  It's a bit of a rushed job, but hopefully it holds!

Perception: 14

Thea offers her rope to Seri, help ing her tie the ropes together 

OOC: square knot time! I can tie a square knot in my sleep, so the DC is probably like 2 or something.

Once again Kairouani has no trouble crossing the river. She, Seri, and Thea gather on the clifftop above William. It is hard to see him without leaning dangerously far out over the ravine, but his shouts help to locate him. Between them, Seri and Kairouani join the two ropes together and secure one end to a sturdy tree. The free end drops down near William, who manages to grasp it without letting go of the vine that was his salvation. With a great effort he pulls his sodden and aching frame out of the current and, with the rope taking the strain, starts to make his way up the cliff. Painstakingly, bit by bit, he clambers up, carefully placing his feet on what look like good, sturdy footholds and going hand over hand up the rope. It creaks under his weight but holds. The knots that Seri tied and Kairouani checked hold firm, and eventually William pulls himself up and over the edge, bruised and exhausted.

The party is finally reunited, on the northern bank of the river, after something of an ordeal to get across it.

It's time to level up!

And then Perception checks all round please (William with Disadvantage)...

Edit: We cross-posted. All good, I'll take 14 as Thea's Perception roll.

OOC: yes! Thanks so much!

Thea almost gives the soaking William a pat on the back, then remembers his current state of sogginess and decides not too. You can take the rich person out of the mansion but you can't take the...IDk what this metaphor is supposed to be,

"that was quite an ordeal. Everyone uninjured?"

As William ascends, Seri gets a faraway look in her eyes, gazing west towards the ocean.  Briefly, the turquoise of her irises expand to fill the entirety of her eyes and her lips part as if gasping, then just as suddenly, she is back to normal, albeit swaying a bit.

She leaves it to Kairouani or Thea to gather and separate the rope and instead steps towards William, making a small gesture with her hand in his and then Kairouani's direction.  Gently but surely, the cold river-water soaking them both streams down onto the ground and away into the river, leaving their garments only slightly damp.  (For the first time in her life, Seri casts Shape Water, to force the excess water out of William and Kairouani's clothing).

So distracted is Seri by this new revelation of divine aquatic wonder, and also by her plans to seek out the metallic object further up by the waterfall, that her ordinarily sharp senses fail her, and she is completely unaware of her surroundings.

Seri Perception:  7 (Natural 1)

 Kairouani  nods that she is fine, then turns towards William, " I apologise for failing to aid you as well as I should have.  I have been out of sorts since arriving here."

Perception- 2

William was down on hands and knees exhausted after the climb up. His wet clothes hung heavy on his body, threatening to pull him down if he did not catch his breath soon. The air he sucked in felt cold in his wet throat and while everything hurt, it meant he was still alive.

He slowly started to undo the buttons on his shirt when he stopped at the second one. His shirt, or rather the water absorbed in the fabric, moved on its own accord. His sleeves pulled away from him but when the fabric neared its breaking point, the sleeves fell back. For about a second a blob of water got pulled out of the shirt and floated in midair before falling onto the ground with a loud splash. His shirt was still damp but no longer soaking wet. William rolled onto his back.

'Thanks.' He managed to say in between deep breaths. 'I owe you one. All of you.' He tried to get a smile on his face before painfully realising that not only his arms were hurting but all his muscles had cramped up.

Level 2 is just going up a level in Fighter. Now William can use action surge to fail twice in the same round ^^
Is maximum hit point taking the average, rolled or the max number?

Perception check with disadvantage: 14

DM roll: 19

Nothing to see here...

The day is wearing on. There is thick jungle between you and the summit that was your goal (I presume it still is), and the heat has reached a peak. The moisture in the ground is turning to vapour, rising in tendrils around the trees. The air is hot and heavy, taking an effort to breathe even when not exerting yourselves. Curious birds flit in and out, observing your presence but not staying long enough to find out if you are predators. The bright colours lend an exotic feel to the scene, for those of you less used to such things. For Kairouani and Seri it is familiar, but neither is in the habit of forging a path into virgin jungle. Most civilisations tend to live around the edges, especially near the bountiful sea. There are dangers in the jungle, from biting insects to venomous snakes (as Thea discovered) to poisonous tree frogs, whose poison is used by some indigenous peoples for hunting. Applied to blow darts, the poison is potent enough to bring down a monkey almost instantly. Still, the jungle is where you have chosen to be, for now, and more of it lies ahead. 

Having recovered your rope and stowed it away, what is your next course of action?

Seri brightens visibly as William thanks everyone, looking every bit the affirmation-seeking teenager that she is maturity-wise, despite likely being older than them all in absolute age due to her elven ancestry.  Her ordinarily sharp attention to her surroundings lapses as she speaks up.

"We should return to the falls to gather the item I believe we all saw?  Then ascend to the hilltop so we can return to Trondro's shore before nightfall."

If all are agreed and nothing external interferes, Seri suggests that William stand on the bank, holding one end of a rope tied around either Kairouani or Thea who then attempt to retrieve the metallic item, while Seri provides them guidance and help with her aquatic/telekinetic Mage Hand.

Thea volunteers to retrieve the item. tying the rope around herself, she ventures out onto the logs, attempting to grab the item.

Acrobatics check (rolled with advantage because of Seri's help :18 plus 4 from guidance.

Strength check to dislodge the item, if needed (with ADV as before): 15 plus 4 from guidance.

Kairouani stands ready to assist with the retrieval if needed...

Thea reaches down into the clear, cool water, feeling the current tugging at her arm. Secure in the knowledge that she is safely attached to a rope, she nevertheless shows off her superb balance. Her hand closes around the object, which is round, and quite heavy. Pulling it clear and looking at it she sees that it appears to be some kind of large coin. It is just a little smaller than the palm of her hand and about a quarter of an inch thick, and seems to be made of gold. It has a different symbol stamped on each side along with some text, which looks like it might be Verdian. 

It is an incongruous sign of human influence on this otherwise apparently untouched wilderness. 

Quote from Tybard >>

Seri brightens visibly as William thanks everyone, looking every bit the affirmation-seeking teenager that she is maturity-wise, despite likely being older than them all in absolute age due to her elven ancestry.  Her ordinarily sharp attention to her surroundings lapses as she speaks up.

"We should return to the falls to gather the item I believe we all saw?  Then ascend to the hilltop so we can return to Trondro's shore before nightfall."

If all are agreed and nothing external interferes, Seri suggests that William stand on the bank, holding one end of a rope tied around either Kairouani or Thea who then attempt to retrieve the metallic item, while Seri provides them guidance and help with her aquatic/telekinetic Mage Hand.

William, having had his experience of the wild waters, nodded and held the rope firmly in his hands. Thea was a good deal lighter and smaller than him so they had an easier time holding on to her than he had previously. When the girl got back to them, she was holding what looked to be a coin, golden and fairly big.

'Looks Verdian.' William said as he leaned in a bit to get a closer look. 'Curious to find it here.' He looked towards the spot where Thea had retrieved the coin. 'Might be a sign there is some civilisation here or maybe some poor sod did not have people to help him out like I did.'

He gave Thea a reassuring nod. 'Gold is gold. Hang on to it: it's bound to come in handy at some point.'

What's written on the coin? And is there a way to ascertain when the coin was minted?

I’ll take a history check on the coin. 

Does anyone read Verdian? 

Seri likely does not read much Veridian and is unlikely to remember any History on a Veridian coin.

However, if she notices the effort one someone else's part, she does does pray to confer Trondro's Guidance on whoever is trying to recall such.