In Russell Spurr's "A Glorious Way to Die" I read a Damage Control officer's account of running by a store clerk passed out dead drunk with a bottle of Johnny Walker Black Label in between his legs minutes before the Yamato went under. Similarly, the author notes that the helmsman, a veteran of the Russo-Japanese war, drank a pint of Scotch before going into battle.
I've always thought of the Japanese forces as being highly disciplined, to point of being beaten by officers and NCOs for minor infractions. Was the culture of alcohol consumption before battle always present in the IJA/IJN or was it a last ditch effort to give soldiers and sailors some liquid courage?
Typically alcohol was only allowed to be purchased when on leave or on rare occasions at the discretion of the commanding officer from the local canteen or the ship's stores. There was also an occasional supplementary army ration of sake, although that was highly dependent on logistics, which were extremely questionable during WWII.
Towards the end of the war, as the war situation for Japan grew desperate, the Kamikaze attack squadrons had a ritual whereby pilots to be sent on their mission would drink some sake. One could argue that it was used to essentially dispel any resistance or last minute reservations by the pilots (as there was definitely a peer pressure element to volunteering in the kamikaze units) but it is more likely just for a show of solidarity, much like "cheers" at a drinking establishment.
In the case of Yamato, pretty much everybody on the ship and her escorts knew it was a suicide mission. Their job in this mission was to act as bait for the US Carrier Air Groups so that a kamikaze attack from Kyushu could attack the American fleet with less resistance. Prior to the sortie, Tameichi Hara, captain of Yahagi, the light cruiser escorting Yamato, mentions how he and the other destroyer captains had a long drinking party that lasted until late in the night, and how the enlisted and senior officers also had one that they visited. They knew their situation was pretty hopeless, so discipline was definitely slacking that day.
As for why Johnny Walker would be in the hands of a store clerk on the Yamato, the battleship Yamato was rather infamous in the IJN for basically being a luxury cruise ship, as unlike most of the other ships Yamato sat at dock in Truk for a good portion of the war. Nicknamed "Hotel Yamato" (after a contemporary chain of Japanese hotels), it was known for having an incredibly large amount of fine food and spirits, as well as an ice cream machine. Likely, the store clerk, realizing that he was about to die, decided that he would die happy and grabbed said Johnny Walker without worrying about naval regulations or repercussions.
Sources:
Hara, Japanese Destroyer Captain
Senkan Yamato
Kuwahara, Kamikaze