Is this From Dusk Till Dawn (1996)...?
From Dusk till Dawn is a 1996 American action horror film directed by Robert Rodriguez and written by Quentin Tarantino from a concept and story by Robert Kurtzman. Starring Harvey Keitel, George Clooney, Quentin Tarantino, Ernest Liu, and Juliette Lewis, the plot follows a pair of American criminal brothers (Clooney and Tarantino) who take a family as hostages (Keitel, Liu, and Lewis) in order to cross into Mexico, but ultimately find themselves trapped in a saloon frequented by vampires.
As mentioned in the block quote above, the plot involves a group of people -- a pair of fugitive criminals, and three hostages they've taken; a preacher and his teenage son and daughter -- who find themselves trapped in a saloon owned and frequented by vampires.
At one point, they make their way into a storeroom in the back, where they find numerous boxes of paraphernalia taken from previous victims of the vampires. They look through these boxes to find anything they can use as weapons, and the boy chooses a Super Soaker-style water pistol which is subsequently filled with holy water, enabling him to burn the vampires.
You can view the part where he finds the water pistol at around the 0:34 mark in the video clip below.