Ishigaki Island is closer to Taiwan than mainland Japan, and is a truly gorgeous tropical location.
The island is small enough to drive around in one day - but the driving is so relaxing, the views so idyllic, and the ocean so blue that I would recommend a three-day visit. There's also a lively bar and restaurant scene, so it's unlikely you'll get bored.
Ishigaki is completely surrounded by a coral reef, so when you fly in, you can see a pale turquoise ring of ocean surrounding the island. Depending on the time of year, the dive sites are frequented by all manner of warm-water sealife, including lion fish, turtles, and manta rays.
There are several dive companies based on Ishigaki, but I chose Viking Scuba Kabira as I wanted a more personalized, multi-lingual diving experience. Anna and Masashi Toyoshima, the husband and wife team, have been working as instructors in the Kabira Bay area of Ishigaki for over five years, and internationally for much longer.
For regular divers, Viking Scuba offer a day of fun dives and full kit rental (which also includes transport, underwater camera, and bento) for just under ￥20,000. You can also choose an "experience diving" or snorkeling package. People who have never dived before or who aren't confident swimmers shouldn't be worried, as you are in safe and experienced hands at Viking. As it's a smaller company, the dives take place at your own pace, and special circumstances or diving interests can be catered for.
We visited during Golden Week (early May) and were able to dive at Manta Point; however, we were a little too early in the year to actually see any of those famous mantas at their cleaning station. That day there was a light/moderate current but it was very manageable. We did, however, see lion fish, sea snakes, clown fish, and many other adorable little sea creatures. Sadly, as is now the case internationally, you could see quite a lot of coral bleaching. This is something that divers in most locations now experience, as the Ocean Agency reports that in the past 30 years alone, 50% of the world's corals have died.
Nevertheless, anyone who loves the ocean will enjoy their time in Ishigaki. The water visibility was amazing and, as mentioned, the colours were spectacular. Most of the tourists I met in Ishigaki were from Japan or other Asian countries - perhaps Ishigaki isn't (yet?) an over-marketed or internationally popular location. Hopefully this will help to keep damage to the marine life at a minimum.