Never swim if you see this beach wave This is why you must stay away from the water if you see ‘square waves’. Picture: Michel Grifon/Wikimedia Commons News by Kara Godfrey, The Sun 19th Feb 2021 12:13 PM Beachgoers who see "square waves" while on holiday should get out of the water immediately, experts warn. 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. Square waves are considered too dangerous to swim in. Picture: Michel Grifon/Wikimedia Commons