The end of June isn't high season, yet the sea-water looks already mudy. In one of my prevoius visites to Lignano, by the middle of July, the water I saw was terrible dirty. I cannot even imagine to swimm in it! The beach was crowded with hundreds of thousands of tourists, most of them coming from the Northern Europe.
Lignano Sabbiadoro isn't good place for those who enjoy in swimming, on the other hand, it offers a lot of amusements, especially during the night.