Please help me make a decision….Ishigaki in April?

I’m taking my daughter to Japan for our first time. We are going to Tokyo, Kyoto and Osaka. We usually round out our trips with a few days at a beach to relax. We are traveling late March – Early April. I was initially under the impression that Ishigaki was like a Caribbean island…warm and sunny all year round. The more I research, the more it seems like it will be too cold to swim, dreary and not sunny and likely rainy and unpredictable. Is that correct? Is there somewhere else (another island in Japan) that would have the sunny/warm weather in April? I was going to take her out of school for a couple days for this extension or should we just enjoy our time on the mainland?