The answer to the crossword puzzle for the term 'SOURCE OF WATER POWER?' is 'OAR' (3 letters). An OAR is a tool used for rowing a boat, and it is also a source of water power. Oars are typically made from wood or other materials such as aluminum, and they are used to propel a boat through the water by pushing against the water. Oars are also used to steer a boat, and they can be used to generate power when attached to a generator.