Swimming in hoodies has become fashionable in a region where they swim in clothes anyway. It is often seen in romantic art, as in Yaoi Anime. The underwater kiss scenes are done in soft casual clothes, like hoodies.
Later the fashion industry picked up the trend. Aliexpress has many swim hoodies for sale, mainly for sun protection when swimming. They come in a variety of styles and fabrics, like cotton, spandex, or thin nylon.