Thanks, evergrn!
The Ferry schedule for boats to Zamami is pretty patchy this April, but the high speed cruisers are up and running. If there are five of you you could go to or from Zamami by chartering a helicopter for 12,000yen each each way. (Regular price on the fast ferry is 3,000 yen and journey takes 50 or 70 minutes.
alternatively, there's Tokashigi Island which is only 30 minutes away (chopper charter presumably a little less too).
Had attempted to go to Ishigaki or Miyako islands to cut out reliance on a ferry, but I'm really glad now that that wasn't possible. Zamami seems to offer EXACTLY what I wanted, a place for my daughter and her grandfather to bond over common interests and passions.
LapChild has spent hours and hours submerged in England's Blackwater Estuary. Her grandfather grew up collecting oysters in Matsushima Bay (he's a fantastic rower).
One of my husband's cousins spent a few days recently at Okinawa's Onna Beach. He'd landed a bed and breakfast deal at the Best Western for 4,000yen a night. It looked lovely, but the resort experience wasn't what I was after - although we're flirting with this kind of experience at Senagajima on our last day.