Kimagure Orange Road: Shonen Jump Special (1985)

Kyosuke, Madoka, Hiraku & friend go on a vacation to the beach, staying in 2 cabins, boys in one, girls in another. It's basically Kyosuke & Madoka get together, & Hiraku flirts with the blond guy friend. Then, Kyosuke & Madoka go night swimming with dangerous results.