There isn't a direct flight from Mykonos to Santorini, almost all domestic flights in Greece are performed through the airport of Athens.
As for boats, the detailed schedule of the ferry companies has not been announced yet. It is usually announced two months before departure, so it isn't sure yet if there will be a direct ferry from Mykonos to Santorini.
Sorry that I din't help much but that is all I know for now.
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