Riptides?

[BlackBellamy]

You get tired out. Bobbing will do nothing except get you swept out further, so you have to swim. And while you're swimming perpendicular you still get swept out, lengthening your return trip. You already used some energy to do whatever activity you were doing before the rip got you, then you use a lot of energy escaping the current, and now you're looking at a hundred, two maybe three hundred yard swim, and it's all waves crashing on you and more weird currents.