Duck Diving is normally a technique for big wave surfing because you can usually float over most waves on your long board and don't need to go underneath.

But if you find yourself in a dire situation and need to escape the monstrosaty of a wave simply grab both sides of the board and place your knee on the board pushing the nose under the surface of the water. Your body and the rest of the board should follow under the wave and then pop out on the back side.

This is called "duck diving" because the technique was developed after watching ducks swim under obstacles in the water.