You should carry at least six lines with you at all times for safely mooring your boat. Two bow lines, two stern lines, and two spring lines. Each line should be 10% longer than your boat, so if your boat is 20 feet long, your lines should be at least 22 feet in length, measured from the eye to the bitter end. If you think that's not possible, then at least make sure that two of them are that long...the ones you intend to use as spring lines. (Of course, there is an advantage to having all six lines the same length...you won't have to try and sort them out when needed to find the right length line.)
When selecting the size line for your dock lines, consider the size of your boat and the size of your cleats. Here's a chart with suggested line sizes by boat size.
Never let too much slack get in your longe line. If your horse moves in as he goes around the circle, pull the line in. Slack in the line can be dangerous if a horse tries to run off.
Never loop end of line around your wrist. Hold loop with your fingers
so that if your horse runs off you can let go of the line.