diving and snorkelling in Ishigaki

We are planning to visit Ishigaki island during April last week. Moreover, I think April would be bit chilly but that's all what we can do. :(. It will be pleasure if we get a chance to do diving and snorkelling. But how do we get the equipments for these activities. I am not able to find anywhere in the website about the availability of facilities. Or do we have to get ourselves?
On top of it, how convenient is it to visit Taketomi and Iriotomi island? Is it possible to cover these islands in one day?
Sorry for asking too many questions.

I will appreciate the reply from anyone who had already experienced Okinawa prefecture.

I would suggest a full day for Iriomote UNLESS you're planning to rent a car. The ferry takes about an hour, then buses to the major sites (waterfalls, beaches etc) don't come very often, so it'll probably take all day to visit with public transportation. Another option is to take a tour, which will usually provide transportation in the package.

Taketomi can be done in a morning or afternoon, but I REALLY enjoyed the island, so I'd say be flexible. Rent a bike or take an ox-cart ride, then give yourself some time. But you can definitely do the island's major points in 2-3 hours.

As for snorkling equipment, it's often included in a tour that takes you out somewhere by boat (for example Barasu-to, which is an all-coral island near Iriomote). Or, around beaches there should be places to rent... although April might be a little early, so there may be less. I'd do a tour to be safe. You can find them through the tourist association/information center when you get to the island, or your hotel should be able to suggest and help you book some.

The Yaeyama islands are great! You'll enjoy your trip!

I went to Ishigaki last summer (and I return soon ...).
I have been diving with manta rays near Kabira Bay by a local club.
http://www.soleilrouge.org/journee-plongee-a-ishigaki

Snorkeling is not as easy as the island of Miyakojima because the reef is quite far from the coast.

I went to Taketomi. It's a beautiful day walks and swimming.
http://www.soleilrouge.org/taketomi-une-ile-paisible

But the best in Ishigaki, it's party time!
http://www.soleilrouge.org/la-fete-a-ishigaki-le-soir

I see,
@scaredragaon: thanks so much for the information that really needs to be noted down. Looks like taking up the tour package could be a better option so that we can do diving. I think of going during the best season by taking suggestion from the tourist information as said by you. Moreover, how much would be the price of the package? any idea?

@fuchan, your pics are mesmerizing. So it means, apart from Ishigaki island, Okinawa (kabira bay) also provide diving and snorekelling?. Sounds exciting. However, unlike you I don't like rain and party at all when I am on travelling mood. :(
So can you Please tell me more about how to book for diving and which is the best season?