Question: I recently ran an encounter on a "restaurant boat" where my players had to obtain info from a noble on board. They successfully bluffed/bribed/sneaked their way on board. Certain areas of the ship were off limits to guest, such as the crew's quarter's, the captain's cabin, and the lower deck with rowers moving the ship along.

My players found a somewhat quiet spot on deck. They proclaimed they wanted to sneak past the guests, who were busy with eating/socializing/etc., into the forbidden areas. One of them cast *pass without trace*, and I asked them to all roll for Stealth. No one got less than 19 on their Stealth check (after modifiers). They then proceeded to "sneak" past guests into the upper areas (not forbidden per se) and then further into "Staff Only" areas.

How do I adjudicate this?

Obviously, they pose as guests, no weapons or armor on, so they would just blend into the crowd and then, in an opportune moment, sneak past a door/curtain/rope barrier. But they are still moving in plain sight of at least a dozen NPCs. The NPCs can probably see them, but they do not *perceive* them.

The same problem would arise for me if they wanted to escape someone following them in a dense marketplace or in a crowded tavern. How can you Hide/roll for Stealth in a crowd of - admittedly uninterested in you - NPCs?

Answer: 
A Charisma (Deception) vs Wisdom (Insight) check
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I'm guessing that you're referring to a *certain location* in Storm King's Thunder. If I'm right, the book suggests the following on page 216:

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>  The adventurers might try to replace one or more of the workers [of the Grand Dame] [...] A character who wants to get aboard in this fashion must **succeed on a Charisma (Deception) check contested by Captain Storn's Wisdom (Insight) check**. A character who wins the contest can board the ship without raising suspicion.
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The informational text on this page is talking about getting on board from the docks while it is tied up between trips, not getting into the *CREW ONLY* sections of the ship while it is underway and entertaining guests onboard but I think the Charisma (Deception) vs Wisdom (Insight) check works for both situations.

It might not be the Captain's Insight they are trying to oppose but that of whomever sees them attempting to access the off limits areas. This could be a crew member or the pit boss Pow Ming).

Even if I'm totally wrong on the campaign, I think the recommendation is still universally applicable.
There are some clear rules concerning hiding in the PHB:

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> The GM decides when circumstances are appropriate for hiding.
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And

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> You can't hide from a creature that can see you clearly. 
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When you find that waiting for an opportune moment to slip past someone is appropriate, that is covered by the rules. It is also worth noting that an opportune moment is probably one when there is specifically no person looking in the exact relevant direction. From your question I gather that the players need to conceal their entry to the forbidden areas and not their presence in general. 

Additionally, Pass without Trace is a one hour concentration spell giving a bonus to stealth for the duration. It does not say that the check must be made immediately. Therefore, you can also demand the check as they enter a forbidden area. In that case, sneaking through the crowd is unnecessary. 

If there is ever, realistically, a moment when no one looks in a specific direction would depend on the number of people in the room (unless a distraction was provided). But this is not clearly ruled by the book and you can decide this as the GM.