Which Okinawa island is best right in the water for snorkeling/diving with sea turtles in late May / early June?

I’m planning a trip to Okinawa late May → early June and my main goal is to snorkel or dive directly from shore or very close by and see sea turtles 🐢.

I’ve been researching and many people mention Zamami Island (especially Ama Beach) as a good spot for seeing turtles and great snorkeling right from the beach.

However:

• I can’t find a scuba diving instructor/centre in Zamami — does that mean it’s not actually that great for diving?

• If I want both awesome snorkeling and easy diving with a good chance of seeing sea turtles, is Zamami the best choice, or should I consider Tokashiki Island, Aka Island, or other parts of Kerama Islands instead?  

Thanks in advance — I’d love recommendations from folks who’ve been there! 🌊🐢