Duck diving a longboard is more technique than strength. You have to remember that you can't just push the board flat under the water. You have to pick a corner on the front of your board and dip/slide that corner into the water first at an angle then push the other side with your opposite hand. Naturally when you get the board under the water your back foot will have to push the back of the board under the water right when the wave is about to pass over you. Lots of practice and you'll get it.
