Nice touches with the free diving cave entrance and diving bell

The pirate in me is happy!
I can't claim credit. That was all [MENTION=6776133]Bawylie[/MENTION].
Pick some pockets, make jokes at Falsetto's expense, and challenge the mercenaries to a drinking contest?
Challenge accepted!
I noticed you used a lot of if the players do Y, then they gain advantage on Charisma check Z. On the face that looks fine, until you look at the specific examples: You help the merc's wounded...then you make a Charisma check (with advantage cause you helped) but on a failure they still treat you with derisive laughter and tell you to go take a long walk off a short pier... ? What the hell?
As a DM, I would instead not even call for the Charisma check and just let the PC's actions influence the mercs on their own merit. Calling for the Charisma check seems superfluous and threatens to invalidate the players' sense of agency in the game world. After all...what consequences really happen if the PCs FAIL the Charisma check?
For example, I'd leave out the hostile/unfriendly/friendly language and imply that if the PCs tend to the merc's wounded that means they're potentially strengthening a rival party. This gives the decision actual gravitas/potency without needing a die roll to give it "false weight." (Hope that makes sense, I realize I have my own gaming parlance sometimes) So it becomes a question of: Do we trust these Wild Hammers enough to help them? OR Do our values of helping others apply even to potential rivals?
My most recent scenarios generally don't mention even a single ability check in them. Like you, I'd rather leave that to the DM to decide during play based on the context I have written and what the players decide to do. In this case, I went back to the "traditional" way this presented, but there are a couple things to note:
(1) The checks all establish uncertainty for a
specific goal and approach e.g. "A tale of boldness and daring and a DC 10 Charisma check might impress the mercenaries..." If the players choose to do something
other than tell a tale of boldness and daring when responding to Jack's call, it might be automatic success, automatic failure, or some other ability check or DC. So it's not "DC 10 Charisma check no matter what you try."
That would invalidate the players' sense of agency as you say.
(2) I don't include the skill that may apply to the Charisma check. Telling that tale of boldness and daring might be Deception, Performance, Persuasion - however the players approach it will determine what skill may apply (if any). I intend to make this a staple of my scenarios in the future. The Basic Rules also say that one approach to ability checks is the DM calling for the ability check and the player applying the skill (or asking to) rather than the DM asking for Ability (Skill) checks all the time. I think the former actually fits the game better. I'm interested in feedback on this part especially.
On the subject of possibly failing that first check, the chances of the PCs failing are slim. With only a DC 10 and the party putting forward a Charisma-primary PC proficient in Charisma-based skills, a roll of 5 or better will get the job done. Add advantage to that and failure is a slim possibility indeed. Chances of failure go down even further for characters with expertise and go up if the party puts forward a less skilled character. Since I expect players to always try to do their best to succeed, I
think we're okay on this first part of the challenge. It also serves the double purpose of getting the players to think about and describe past adventures which is always useful and fun in my experience. As to why the mercenaries would still rebuff the PCs even after being healed, well, they're kinda dicks. Or at least that's how I see them.
Failure in this challenge means a few things: Your rivals are still hostile. This means no information about their plans (short of dealing with Golly) and no easy access to their diving bell which is very useful for accessing the undersea path into the ruins. It could also mean they try to jump you after you leave the ruins. (This is noted later on in the adventure. There's also this whole side deal with Captain Argh onboard the Wild Hammer's ship and his dispute with Jack Wallop, but that's a different challenge.)
I haven't been DMing lately, but last year I successfully ran a structured
interrogation encounter in 5e (inspired by 4e's skill challenges) that played very well. Here it is:
http://www.enworld.org/forum/showthread.php?467592-Interrogation-skill-challenge-in-5e
I'd describe Dealing with the Wild Hammers as a
negotiation encounter, since the stakes are about gaining info/resources/access from NPCs. IME such encounters benefit from a minimum of dice-rolling. Some dice-rolling is fine of course, but the main outcome of the negotiation should be based on decisions rather than dice.
Thanks, I'll check out the skill challenge you linked and see if I can incorporate some of the ideas. I'm a big fan of D&D 4e's skill challenges and the Wild Hammer's interaction challenge is basically the same idea but not structured in exactly the same way. There are
effectively primary skills (Deception, Performance, Persuasion) and secondary skills (Insight, Medicine), plus set number of successes and failures that determine an outcome. In this case, three successes with primary skills results in the mercenaries becoming friendly. Two successes results in them becoming indifferent. One or zero successes results in them remaining hostile. It falls short of a Complexity 1 skill challenge in that there are only 3 complications to overcome instead of 4 and the first check is a "gate" for the rest of the challenge. There's even something of a fail forward built in since PCs gain glean at least some information from the interaction regardless of the level of success or failure.
I appreciate the feedback! Good stuff for me to think about.