The mysterious theft in the Nigerian ship

Question

The Nigerian captain Abomeli Iweala was carrying his ship across the Atlantic Ocean, taking with him five foreigners who worked for him. One day the alarm bell rang, and when the captain arrived he missed a precious gold watch that had been inherited for many generations. Immediately suspicious of the foreigners, he asked his first officer to interrogate them. Each foreigner had a distinct nationality, slept in one of the basement's individual cabins, wore a different outfit, had a preference for a unique drink, and at the moment the alarm rang, was working at a different role.

Your officer has provided you with the following information:

1. The Thai was in the 'Neptune' cabin.
2. The Brazilian said he was cleaning the basement at the time of the robbery.
3. The Argentinean drinks sparkling water.
4. The 'Coral' cabin is to the immediate left of the 'Beluga' cabin.
5. Whoever stayed in the 'Coral' cabin liked coffee.
6. Whoever wore jeans and a T-shirt was sleeping in his cabin at the time of the robbery.
7. Whoever was in the 'Narval' cabin wore a jacket.
8. The man in the middle cabin drinks tea.
9. The Frenchman was in the corner cabin on the left.
10. The blue-robed man stood in the cabin next to the one who was cooking at the time of the robbery.
11. The man in the cabin next to the man wearing the jacket said that at the time of the robbery he was fixing the radio.
12. The man who wore a red coat liked to drink beer.
13. The Korean was wearing an overcoat.
14. The Frenchman was in the cabin next to the 'Marlin' cabin.
15. The blue-robed man was in a cabin immediately besides to the man who drinks whiskey.
16. A man who did not drink beer said that at the time of the theft he was adjusting the flag of the ship, which was upside down.

Of possession of these informations, the captain realized that one of the men lied, and therefore had stolen his watch. What is his nationality?

Answer 1

As CyberianRat noticed,

the Nigerian flag is symmetrical, so the man who claimed to be turning it the "right way up" was lying.

By filling in the following logic grid using the information provided by the officer (with this online tool) ...

... I found the identity of the "flag fixer", and determined that

the Korean was the watch thief.

Answer 2

Whodunnit:

The lying thief was the Korean coffee drinker wearing the overcoat,
and was staying in the 'Coral' cabin.

GIVEN:
person in Coral likes coffee, and middle cabin likes tea, so Coral cannot be middle cabin.
so cabin must be like this -

-?-     Marlin  -?-     Coral   Beluga
french
                tea     coffee

Thai is in Neptune, so

first cabin cannot be Neptune (Frenchman is there); that leaves Narval.

We know (as found by CyberianRat) that

The person who said they were fixing the flag is the liar, as the Nigerian flag cannot be flown upside down.

Person in Narval wears jacket.
Person in cabin next to jacket-wearer was fixing radio
   (and is not the liar, as that contradicts the liar's alibi).
Now we have -

Narval  Marlin  Neptune Coral   Beluga
french          thai
                tea     coffee
jacket
        radio

Argentinian drinks sparkling water

so can only be in Marlin or Beluga (other cabins spoken for either by drink or by nationality)

Brazilian said he was cleaning the basement, and is not the liar, so

cannot be in Marlin (Marlin was fixing radio);
Brazilian can only be in Coral or Beluga (other cabins claimed or excluded)

Red coat wearer drinks beer so

can only be in Narval, Marlin or Beluga (other cabins spoken for by drink)

Then

Korean can only be in Marlin, Coral, or Beluga (other cabins claimed)
Whiskey drinker can only be in Narval, Marlin or Beluga (other cabins spoken for by drink)
Blue robe wearer next to whiskey drinker, so can only be in Coral, Neptune, or Marlin (others excluded by clothes or drink)

Blue robe wearer is next to cooker
Korean wears an overcoat

From all this, we know that

If Brazilian in Coral (cleaning basement):
   Korean/overcoat in Marlin or Beluga
   Argentinian/water in Marlin or Beluga, whichever is left
   red coat wearer drinks beer so is neither Korean nor Argentinian; can now only be in Narval
   --XX Narval wears a jacket, a contradiction.

So we know that

Brazilian in Beluga (cleaning basement), leading to:
Argentinian/water in Marlin
Korean/overcoat in Coral
whiskey drinker in Nar,val or Beluga
  but blue robe can only be in Marlin (Coral is overcoat; Neptune can't be by whiskey drinker now)
so whiskey drinker in Narval
beer drinker/red coat in Beluga (only drink spot left)
jeans/sleeper in Neptune (only clothing spot left)
cooker must be in Narval (must be next to blue robe)
so liar must be the Korean, in Coral

And the final grid:

   cabin:  Narval  Marlin  Neptune Coral    Beluga
  nation:  french  argent  thai    korean   Brazilian
   drink:  whiskey water   tea     coffee   beer
 clothes:  jacket  robe    jeans   overcoat redcoat
activity:  cooker  radio   sleeper fix flag basement
  STATUS:                          liar

Answer 3

The liar is

the man who doesn't drink beer (the last one in the list)

because

the flag on a Nigerian ship looks the same upside down.