In season 8 Episode 21 Kevin is trapped in Crowley's alternate dimension of Garth's house boat... Kevin hears a knock, goes to open the door of the house boat and sprays "Dean" (whom is a Demon in disguise) in the face with a water gun, for not using the "secret knock."__ Then "Sam" jumps out and Kevin sprays him with the water gun as well.... One can only assume Kevin has Holy Water inside the water gun, otherwise spraying them w/ plain water would make NO sense. So why are the "Sam" and "Dean" demonic impostors... immune to holy water??? -I've considered that Crowley is running the alternate dimension but if Kevin has rosary, water, and a prayer... Viola! = Holy Water...
Kevin thinks he has holy water but he really doesn't.
Everything in the pocket dimension was put there by Crowley and he's too smart to put real holy water in there and then send in his demons.
Or... since Kevin knew they were demons, he consciously sprays normal water just to make sure Crowley keeps thinking, Kevin knows nothing about his plans.