Being a scuba instructor, I take an interest in stuff like this. If you dive, or want to learn, Japan has some pretty cool options for you, more often than not with English-language assistance. And it's not just limited to the white-sand paradise of Okinawa. You can don your drysuit and head to Hokkaido, or just a 5-7mm and hit the Eastern coast of Honshu. In fact there are around 2,000 individual dives sites around Japan.
So, here are the five choicest choices:

