Beach Island in April

Hello! I'm starting to plan a 3 week vacation to Japan. 😍 It will be our third trip to this wonderful country! ♥️ We'll go in April or May (I'd like to enjoy the sakura one more time, but I know it woll be crowded).

What would be the best destination for 4-5 days in the beach?

We are thinking to go again to Miyakojima, which we've loved the last time. I've checked in the site weather-and-climate below apparently April would be better than May. But will the temperatures be high enough to go to the beach to swim ( and make some snorkeling)? Many thanks for your inputs! 😊