Sören S.
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There are plenty of nice beaches/beach areas on the eastern part of Sardinia. The beach in Cala Gonone is okay - pretty stony (but small and tiny stones). If you have something like beach-shoes or diving-shoes you should take/use them !
From water-perspective it is going into deep very quickly.
Since it is pretty close to the promenade, you will get drinks and good there.