Remember the aeroplane does not know it's over water. Take sensible precautions though - wrt weather etc. Do it on a good day.
Kiliekrankie at the northern end of Flinders Island is a beautiful place. Well worth the visit but arrange accommodation in advance.
Tassie itself is stunning, and well worth the visit to just about anywhere on the island. Make sure you allow the time for good weather to do a scenic flight over the west coast. Take care though out west, and maybe ask a local pilot for advice.