As waves pass overhead, they create a back-and-forth movement of water called surge. The effects of surge increase when waves are large, but also decrease as you increase your depth.

Strong surge is capable of moving your body back-and-forth while you're underwater, and you'll quickly discover that it's nearly impossible to swim against it. To avoid overexertion, it's best to rest as the surge pulls you back, then swim forward as the direction of the surge shifts to the direction you wish to swim.