Okinawa \ Miyakojima weather in march?

So I booked a trip here with my family ( two adults, one 4 year old) Miyakojima a few months ago. The goal was a break from Tokyo for a few days.
Reading quickly on the internet i found the "Good weather all around" and i was hoping for some nice weather for a relaxing family trip to the beach.

But looking now the weather in late March , is newer good and the hotel pools are mostly closed.
Temperature is also just about ~20c witch is not the best.

Is the weather there relatively bad that time of year?

And do any one have any recommendations for hotel with a heated/open pool in March? I sold it in to my wife as a beach holiday..