Kayakers are especially prone to shoulder dislocations. Due to the nature of the sport, the muscles around the back of the shoulder are built up giving an unbalanced pull on the joint. One keen racing paddler I knew popped his shoulder for the third time merely by sitting down and raising his arm to the back of the sofa, so the biased pulled of his muscles ball out of the socket. So it's a good idea to work on the muscles in front of the joint to compensate.
Skillfull paddling helps prevent dislocations so get good instruction and practice, especially high braces. Basically, leave a bent elbow between the paddle and shoulder to allow some give if the paddle snatches. In the event of a dislocation, most First Aid manuals will tell you to immobilize the effected arm in a position most comfortable forthe victim, and get them to medical aid as soon as possible.
This is not always practicable, especially if paddling out is the only option. If the casualty decides to go ahead with attempts to relocate the shoulder, the sooner it is done the better. Never try without the victims' permission!
One method is to get the victim to bend forward from the waist while supporting their chest. With the effected arm hanging down, turn the palm so it faces forward and pull with a steady, constant pressure directly downwards on the arm, slowly bring the arm forward toward the head.
Another is to get the casualty to stand straight holding the effected hand in front of their body with their good hand, then to slowly raise it as if zipping their jacket, keep going up until successful.
A third is to lay the victim face down on a boulder or tree trunk, with the injured arm hanging down. Tie a weight of about 2kg to the arm, a helmet full of stones would suffice. The weight is increased to about double this, but reduced if pain increases. The casualty is then left untilthe bones clunk back into place.
If the shoulder is successfully relocated, the arm should be immobilized unless it is necessary to use it, and professional help sought as soon as possible. Check the effected limb regularly for a change of colour and a pulse to ensure no blood vessels have been trapped, and move thelimb if necessary.
It is recommended that you attend a First Aid course as this article is meant as a reminder of some of the basics. For more, information I would suggest reading White Water Safety and Rescue by Franco Ferrero, or visiting these websites - http://www.haze.demon.co.uk or http://www.rivers.org.nz