It is a rip current, which can appear RANDOMLY along the coast. This area will have a darker color because the water there is deeper than normal, the water surface is calmer and the waves are smaller… Once you step in here, the water will sweep you out into the middle of the sea at such a fast speed that an Olympic athlete is unlikely to be able to swim back. This is also the cause of 80% of drownings at sea.

If you get swept away by this current, the first thing to do is stay calm, then swim horizontally to escape, don’t try to swim towards the shore.