Isumi, Japan offers 6 beaches perfect for sunbathing, surfing, swimming, and more. Isumi is known for the Isumi Railway, which offers scenic rural train rides through the picturesque countryside of the Boso Peninsula. The best time to travel to Isumi, Japan, is in late spring (May), when the average air temperature is around 18°C (64°F) and the water temperature is approximately 17°C (63°F). Sandee provides detailed maps, photos, and reviews from fellow beachgoers to help you Choose Your Beach™ and plan your perfect beach vacation!