Complex Cleat Knot
Try a complex cleat knot for a secure tie. A complex cleat knot is another option if you want to really secure the dock lines, though it will take more time than a simple cleat knot. Start by looping the line once around the bottom of the cleat. Then, slide the line over the top of the cleat and wrap it around the opposite side of the cleat. Raise it over the top of the cleat and loop it under the first arm to form a figure 8 shape. Make a small, underhand loop and place it over the first arm. Tug the end of the rope to secure the knot.
You should have a nice, tight figure 8 shape on the cleats when you form a complex cleat knot. Use a complex cleat knot for all the dock lines, if you decide to use it.