Osakana wa Ami no Naka

Five year old Matsui is playing with his friend and his friend's dog. Suddenly, they both notice a blue-haired older boy staring at them named Yuuji. Matsui doesn't know him but is surprised to hear Yuuji say, "I want a dog." Several years later, Matsui is already in school (but I don't know which year) and it is revealed that he is third in school's swimming contest (in boy's group). His best friend is Yoshino, who from what I've noticed has more than platonic feelings for his best friend. (Source: ANN)