There is a reference to the "Adriatic water" right before Paul dies. What is the significance of this and another reference to water in the story "Paul's Case"?
The tragic adolescent Paul, who has something "sort of haunted about his smile," rejects his life with his room and "its horrible yellow wallpaper" and the "ugliness and commonness" to his neighborhood; indeed, he has
a shuddering repulsion for the flavorless, colorless mass of every-day existence; a morbid desire to cool things and soft lights and fresh flowers.
In the world that Paul inhabits "the natural nearly always wore the guise of ugliness." So, he felt that "a certain element of artificiality" was necessary for beauty to thrive. In the theatre, Paul has this world of attractive artificiality,
his bit of blue-and-white Mediterranean shore bathed in perpetual sunshine.
Thus, for Paul the aesthetic is blue and beautiful, while the quotidian is "flavourless, colourless"; so, when Paul's crime is discovered and his father is due to arrive in New York for him, Paul decides that he has lived the life he should. Therefore, there is nothing to do but not return to the monotony of Cordelia Street. Although Paul is in despair, as he jumps before the train, through his brain flashes "the blue of Adriatic water", an image that indicates Paul's undying ability to dream and appreciate the aesthetic.