Three boating knots you may find useful:

Reef Knot

The reef knot is used to tie the two ends of a single line together such that they will secure something, for example a bundle of objects, that is unlikely to move much. In addition to being used by sailors for reefing and furling sails, it is also one of the key knots of macrame textiles.

The Bowline Knot

This knot is used to make a loop or eye that will not slip but is easy to untie.  However much load is put on it the loop will not close but is easily released when the load is taken off.  This is why it is the only knot used by climbers for tying a rope around themselves.  On boats it can be used for putting an eye in a mooring rope so that it can be quickly put over a cleat or bollard on board, or it can be used for joining two ropes securely; a bowline is tied in the end of each, the second one through the first.  This is useful if you need to tow someone, the mooring lines can be used to make a longer line.  This knot needs lots of practice as its not the easiest to tie.  See how to make this knot below:

Hitch Knot

Is used to join two ropes together or a rope to itself.  If done correctly a knot will hold shape regardless of it being fixed to something else.  A hitch is used to fix a rope to another object, such as a carabiner or pole, and relies on that object to hold.  You can see this easily by tying an eight follow-thru onto a carabiner.  Do the same with a clove hitch.  Now take the carabiner away and see what happens – it will fall apart.  Some sources classify a hitch as a class of knot but the general distinction remains the same

A simple overhand knot, where the working end of a line is brought over and under the straight part.

A clove hitch is two successive half-hitches around an object.