I didn't really learn how to tie knots until I had paddled and power-boated for years, then we became proud slaves of a sailboat. I urge any boater to learn at least the basics. You will use these knots every time you go boating - because a properly tied knot does not slip and is easy to untie!
I do not include how to tie knots as other do it better - except for the quick Bowline. Someone showed me how to tie this years ago and I'm forever grateful. A "round turn and 2 half hitches" is not usually considered important for marine use but in an emergency I once had to fasten a line to a "lost" dinghy in large waves with one hand, and this was the only knot I could manage. So here are the basic knots in order of learning (in my opinion). You can learn them all in 20 minutes but practice often until they are automatic. You won't be sorry.
The Highwayman's Knot (Mooring Hitch) is one of my favorite knots. It is used to temporarily tie the boat to a ring on shore with the end of the line led back to the boat - a sharp tug on the line releases the knot (hopefully). It allowed us to tie up our stern ladder so it could be released by a person in the water! Could save your life if you fall overboard and your ladder can be dropped into the water this way.
This knot creates a loop at the end of a line, and has many uses. When properly tightened, it is strong yet easily untied. The rope's end can be passed through a ring, around a spar, or through a fitting. It can make a quick loop to throw over a bollard or shore cleat. If you don't know how to tie a sheet bend to attach two lines togehter, just tie a bowline at the end of each line passing through each other.
This knot forms as if by magic whenever you do not stow or coil rope carefully. If you want your own special knot, this is it. But I have to warn you I did not name this knot. I grew up in Saskatchewan, and realize there are fewer boats there than other provinces endowed with thousands of lakes and rivers. Still, I laughed when I saw this diagram.
When a rope is used on a boat for a particular purpose, it is called a line. Many lines have special names. Sailboats have the same lines used by powerboats (dock lines, anchor lines, flag halyards), but they also use lines for sailing, such as halyards, running rigging, topping lifts, furling lines, sheets, and more.
Choosing the type of line for different purposes: