I adore Corsica and especially Calvi. Its clean, pretty and sophisticated with many quality restaurants, and the beach does not let this small town down.
The beach is only a 20 minute slow walk from the fortress, passing the marina on the way and the many many flower displays that filled the street with perfume. The sand is soft and clean and the water is as clear as tap water and very calm. The sea is not deep at all, and you must walk out some distance to reach waist high. It is not rocky underfoot.
There is a number of beach cafes all operating sunbed facilities. We laid down at the first one we come to and 2 sunbeds and 1 umbrella cost 12 euro (a cappuchino was 3 euro). I was told a little further up 2 beds and umbrella was 15 euro, so check along the beach for something that suits you.
As the beach is 6km long it never feels overcrowded or busy. I would recommend Calvi for a visit and hope to return again.