Nagasarete Airantou

Ikuto Touhouin is a young boy that was thrown out to sea when he was onboard a ship and there was a vicious storm. When he comes to, he discovers himself on an island named Airantou. As he wanders the grounds he notices that something is strange... but not much thought is put into it since girls (lots of them) crazily chase him 'round the island.
Ikuto searches for an escape and starts heading out of the island by boat, however, the island is surrounded by whirlpools that will definitely kill. Finally admitting that he won't be able to leave the island alone, he decides to settle down.
... and that's when he finds out that the island is only inhabited by women.