Porto Limnionas is by far the best swim on the island. It was recommended to me by a greek local. The water color is stunning, with shades of green, blue and turquoise. It can be very crowded if you don't come early to get some sunbeds, as they are in a limited number. There is only one tavern there and a line can build up.
But the water is sensational! Swim in and around the caves carved by the water and you will be hit by currents of cold water coming from the springs inside.