Answer:
a paddle
Step-by-step explanation:
Using a "paddle" is very important in order to move/propel a boat. Paddling creates a force which goes against the water. This force is faced by an opposite force that is equal and that which allows the boat to move forward.
So as you push the water asides, the boat accelerates. Such technique is deemed efficient when using the boat. Not following the proper technique will not move the boat.