I have been vacationing this past week on San Juan island on the west side of the island. My husband I went out kayaking (on a guided tour, thank goodness) and had a great time. We saw harbor seals, a giant jelly fish and beautiful scenery.
While we were kayaking, I noticed the movements could have caused me some shoulder pain, but I used positions which kept my upper arms down lower than shoulder height, and avoided a rotator cuff strain or impingement.
As you can see, shoulder impingment and bursitis can be caused when the arm is at or above shoulder height by jamming the ball of the shoulder up into the acromium.
The correct arm stroke is to keep the elbow a few inches below the shoulder, and push forward with the elbow while pullling back the the other arm. Use your oblique abdominals to help power your stroke to keep the strain off the shoulder, too. Here's a video from Youtube about proper kayak technique