 Apart from their blue colour , the mythical creatures look much like humans , and are about the same size . They have the power to create storms , but when the weather is fine they float sleeping on or just below the surface of the water . The blue men swim with their torsos raised out of the sea , twisting and diving as porpoises do . They are able to speak , and when a group approaches a ship its chief may shout two lines of poetry to the master of the vessel and challenge him to complete the verse . If the skipper fails in that task then the blue men will attempt to overturn the ship and capsize it . 
