How much distance should there be between the parallel lines I walk on the beach in order to cover the entire survey site or transect (in other words, what is a good “line of sight” for surveys)?

Your line of sight should be determined based on your eyesight and survey experience, ability to detect debris, substrate grain size and color, size and abundance of natural debris, and typical abundance, size, and composition of marine debris. One meter is a good starting point. Survey partners could hold opposite ends of a rope with a ribbon or marker tied in the middle, with each surveyor assigned the area on their side of the rope as you walk parallel to each other.