It is a form of jousting where the adversaries carrying a lance and protected only by a shield stand on a platform on the stern of a boat. The boat is propelled by oarsmen or, in some cases, a motor may be used. The aim of the sport is to send the adversary into the water whilst maintaining one's own balance on the platform.
The jousters stand on a wooden platform on their boats. As the two competing boats draw level with each other, each jouster, protected by their shield, uses their lance to push his opponent off the platform and into the water. The exact rules of the contest vary from region to region and country to country.
The practice dates back to the twelfth century C.E. in Europe as a sport, with a deadlier version dating back to ancient Egypt.
Chivalry is not dead. It has joined the Navy.
Additional source: Faith In The Good