A coxswain is an obnoxious, sadistic twerp with Napolean complex that sits in the stern of the boat.
But besides that, a coxswain is an essential part of the boat, perhaps just as essential as the rowers themselves.
A coxswain needs to be small. A coxswain needs to be loud. Those are the only two real requirements for being a coxswain. Sure a little boat knowledge would help, but really, that's all there is to being a coxswain. Because of the fact that they just yell and steer, coxswains need to be light because they are just dead weight in the boat. A coxswain needs to motivate a crew. In the last 500 meters of the race, when all of the rowers are exhausted, it is the coxswain that the rowers depend upon for the motivation and the determination to win. The coxswain makes them add that little extra bit a of strength that they find deep down inside of themselves.
Coxswains' primary job is to keep the boat moving straight. If a boat can't stay in the lane, it gets disqualified. Coxswains steer the boat by making minor corrections in the direction of the rudder. If a boat is leaving its lane, the official behind it will wave a flag at the coxswain.
During the race, besides steering, coxswains need to keep their wits about themselves and control the pace of the boat. They do this by using their cox box, which tells them the current stroke rate. If it is off of race pace, then they will tell the stroke to control the rate and bring it back to race pace.
After a victory, it has always been tradition for the rowers to throw the coxswain into the river, even thought this is not always to the coxswain's liking.
For more information about coxing, go to The Coxswain's Locker.