The Mystery in the Net (1) Meitantei Conan : Season 1 Episode 246

Professor Agasa takes the Detective Boys and Conan at the beach during school break while Ran and Sonoko also happen to be on the exact beach. At night, all of them ate dinner in a hotel together with the lifeguards they met that morning. The man the lifeguards were supposed to meet didn't show up so they tried calling him but the only sound that could be heard was the sound of the waves.