Kozushima Island of the Izu Islands

Kozushima Island is great for snorkeling since the surrounding clear water is blessed with a wide variety of both temperate and tropical marine species such as colorful tropical fish, hard coral, soft coral and sea turtles due to tropical water brought by the Kuroshio Current. There is also a group of dolphins living in the surrounding waters all year, and Sperm whales and Minke whales are often spotted. For those who like hiking, the scenic trail to the top of Mt. Tenjo is highly recommended.

Senryo-ike, a crater filled with sea water linked to the sea is a great spot for snorkeling even when there is a typhoon approaching as it is completely sheltered from waves.
Just like any other Izu island, Kozushima is excellent for fishing and spearfishing.

There is also a beautiful campsite run by the government of Tokyo.

About the Tokyo/Izu Islands:
Apart from the Ogasawara Islands, the Tokyo Islands (also known as the Izu Seven Islands) are a great weekend getaway for Tokyo locals. Usually people aim for the Friday night overnight passenger ferry which arrives early on Saturday mornings, where they can enjoy nature activities for the weekend. It is usual to take the high-speed jet ferry on the return to Tokyo on Sunday afternoons, or a flight depending on the island. All these islands of Tokyo are excellent for anglers of all skill levels as the surrounding sea is very rich in fish due to the Kuroshio Current and rugged underwater topography.