I think it means "don't swim in this water, because a chunk may fall off the glacier and cause a small tsunami that will interfere with your ability to keep afloat".
That said, if a chunk did fall off the glacier and cause a small tsunami, it wouldn't just be the people in the water who'd have cause for concern.
Why do people in these photos always have their arms in the air? I'm not a good swimmer myself, but even I know that attempting a butterfly stroke under these circumstances is a bad idea.