We spent 4 nights here at the beginning June. We were the beneficiaries of a gracious English-speaking host at a small hotel where every room has a view of the nearby cove and comes with 3 meals a day. For a first-time visitor to Japan jumping right in to the deep end (visiting a small island off the coast of the southernmost prefecture of a country where English is often a distant 2nd and the value of the Yen does not favor the USD), this made our trip.
The locals are generally friendly, and even at worst are still accommodating of misunderstandings by foreign tourists.
The water is blue and clear and warm. Be sure to snorkel at either Furuzamami or Ama beaches. You'll be rewarded with colorful, bountiful fish and coral reefs and, sometimes, see turtles.Rental equipment (masks, fins, wet suits, towels, chairs, inflatables, etc.) are available for rental (we paid, each, 300¥ for towels, another ¥1000 for fins plus snorkel mask, but didn't rent a life jacket).
There's also several viewpoints dotted around the island, and are fairly easily accessible by bike (manual or e-assist) or scooter rental (assuming you have your IDP). Our host was kind enough to drop us off.
Also good to know, there's an ATM that accepts foreign cards in Zamami village.
And that brings me to my caveat: the purchasing power of your exchanged currency is not on your side. At the time, the exchange rate of our dollars was 113:1. But the cost of things made that feel like 120:1.