Because the phenomenon is usually associated with strong and powerful rip tides. Swimming or surfing in the middle of a cross-sea is not something you should be doing, even if you're an experienced swimmer or wave rider. Square waves can also cause boating accidents and shipwrecks.

Why are square waves so dangerous?

Also known as cross seas, they may look pretty but are actually extremely dangerous - and have even caused shipwrecks. Square waves can be found out in the open ocean as well as near the coast, and are formed by waves moving in opposite directions. This happens when two separate weather systems collide.

What happens if you swim in square waves?

“If you swim in square waves your life is in danger,” says one video. ... The pattern is known as a cross sea, and occurs wherever where waves from different weather systems meet each other at right angles. The waves come from far out at sea, travelling for some distance without the wind that raised them.