Proper posture is necessary to balance and executing good solid paddling strokes.  If you have been paddling awhile, you will see/hear many boaters that say they flipped because they caught an edge.  This is usually caused by paddling in the barco-lounger position (leaning back when they see something disconcerning like a hole).  This dips the back portion of the boat underwater where the water currents can control the boat.  You will also hear some boaters brag about a very aggressive leaning forward paddling position.  If you lean too far forward, your body movements are severely restricted - especially torso rotation.  Sit up straight and learn to use your hips as shock absorbers and maintain that upright posture.  Here is a nice short video on Paddling Posture.   Here is another nice article: Body Language.

When I place beginner paddlers in the water for the first time, I talk to them and rock their boats from side to side to see if theyy are comfortable.  I then have them rock their boats using their hips on their own so they can get a feel for the boat stability and how to balance the boat keeping their torso upright.  This process slowly builds confidence in how to balance themselves in the boat.