Lots of people stern tie up here without floating lines but a floating lines can make for easier retrieval. Here is but one example from Amazon in 3/8 to 5/8 sizes and various lengths. Most people tie off to a cleat go around a tree or whatever and come back to the boat to tie. This way they can depart without having to go ashore, so make sure you get a long enough length.
https://www.amazon.com/PolyFoam-Floa...YPY&th=1&psc=1