When to visit? Beginning of April, end of May/early June?

I’m planning a trip with my family to Japan and want to visit Okinawa. My mom hates the heat (where we live it gets in the low 90s with 60-70% humidity), so I’m leaning towards April. But three of us on the trip want to engage in scuba and snorkeling.

Is there a big water temp difference between start of April, last week of May or the first week of June? We are also limited on when due to a child still being in school.

Thanks!