You're on a Boat . . .

[pauljathome]

Does anyone else feel this logic is sound or believe there is some hole in it?

Its sound enough to raise suspicion but certainly not guaranteed to be correct. When questioned people don't exactly always answer perfectly, people DO misspeak, jump to conclusions, etc.

 

[SnowleopardVK]

First thing I'd do is start abusing magic, and anyone's right to not have their thoughts sorted through.

...Okay, assuming either no magic, or not enough access to it, I'd start with detaining and isolating everyone present, investigate the scene of the theft to find any clues, and then move on to questioning. Give the ones with the most stable alibis some quick attention first, then move on to the more suspicious ones. With any luck I'd sense a motive somewhere in 30 questionings, get enough of a lead to allow for a proper, "official" detainment and investigation once we reach shore, even if the original lead is a red herring.

Once at shore enlist authority help if at all possible, make note of each suspect and pay VERY close attention to anyone who decides to sneak off.

 

[Dannyalcatraz]

Is this another one of those Costa Crociere/Carnival cruises?

 

[MarkB]

How do you proceed?

Claim that I need to consult the authorities on the mainland, then make off before anyone realises I nicked the necklace.

 

[CAFRedblade]

The old "he doth protest too much", concept, he either has a perfect memory, or has attempted to craft the perfect alibi.
Although not enough to convict, it's definitely suspicious, and would lead to further questioning, and searching of gear.

Oh, you were here on the last night's party, at exactly this time, with these people... over the previous days, please give an exacting detail of where and how you spent your time aboard ship...

 

[Dannyalcatraz]

It's an insurance fraud scam- the first night necklace was a fake, and ditched overboard later that night.

 

[Dioltach]

It's an insurance fraud scam- the first night necklace was a fake, and ditched overboard later that night.

I like the way your mind works. Smoke and mirrors ...

 

[Man in the Funny Hat]

So, you're on a boat (sans flippy-floppies) for five days with a bunch of aristocrats and a crew. As far as anyone seems to know, no one has boarded or left. There was a big first night party and there has been a final night party. At the first night party, one of the high ranking honored guests wore an expensive necklace which was then stashed in the cabin at the end of the first evening and presumed safe. Several days and nights of lounging around intervene then there is the final night party which is enjoyed by all.

The next morning while packing the honored guest discovered that the necklace is missing. You are appointed by the patron to investigate and the boat will arrive in port in a few hours.

Tight schedule and with no authority to even detain suspects without sufficient supporting evidence I would hope I'm being well paid regardless of the outcome of the investigation as it's likely to end in failure (and since the PC's DID toss the necklace, even if I nail them to the wall circumstantially, without them having the goods on their persons, in their belongings or stashed aboard ship it IS a failure.)

Otherwise I proceed on a few basic, detective story tenets - who has motive, means and opportunity to commit the theft? Eliminate the impossible, what remains must be the truth. And, the simplest explanation is usually the correct one. Everyone is questioned (separately I hope) and is asked for as detailed an explanation of their comings and goings from the time of the last appearance of the necklace up to at least the point where it was noticed to be missing if not to the present. Everyone is asked who THEY think may have done it, why, and how. Get them suspecting everyone else and thus working FOR me to narrow the possibilities.

As you question the thirty guests and crew about half are immediately offended at being accused and will give no further information stressing only their denials of guilt.

Doesn't mean one of them didn't do it, but without incriminating testimony from someone or evidence of some other kind I'll initially write them off.

Another seven have clear alibis for the full voyage either without access to that compartment or being in sight of yourself, the patron, or the honored guest for every portion of the trip when the guest was not in their cabin or wearing the necklace at the first night party.

Just because you're not supposed to have had access doesn't mean you didn't or don't still. They can also be complicit in the crime even if they didn't lift the necklace personally. In particular, I'm thinking that at least two of this group could be providing each others alibis. [A says he saw B on deck all through breakfast on Thursday, B says he saw A when he went on deck at that time and was still there when he returned to his cabin, etc.] I'd likely draw up as detailed a schedule as possible of everyone's alibis throughout the voyage and see if there's some timeframe like that to draw suspicion.

Four more deny guilt but seem understanding of the need to investigate and do not begrudge your questioning.

And I can only assume that they provide no especially useful information despite being cooperative.

Two others (separated at the time of questioning) deny guilt but suggest each other might have something to do with the theft. One denies guilt and says he was at the closing night party where anyone could see him.

Suspect #1. Having not stated WHEN the necklace disappeared it is odd to mention the closing party and not some other time, or indeed to mention some specific time period AT ALL. On the assumption that the suspect is attempting to mislead with regard to the time frame of the actual crime I would look particularly closely at his verifiable movements prior to the party, and/or between the time of the party and the time of the discovery of the crime. Who's providing his alibi? Are they mistaken or actually lying about the suspects location?

And the last refuses to even accept the premise claiming the honored guest obviously has some reason for staging the theft.

Which, of course, is a classic detective story reversal. Again: motive, means, opportunity.