In Celtic folklore the Na Fir Ghorma are a very rowdy group of Mermen with the ability to cause sea storms. Though these mermen are most commonly encountered in the waters around the Outer Hebrides and the Minch, they inhabit the entire North Atlantic, originally only around Europe, but as the people from this area migrated to North America so did the Na Fir Ghorma, so that now they can be found from Scotland to New York.
They, however, prefer the cold and temperate waters of northern latitudes and can't be found in tropical latitudes, The Na Fir Ghorma, also known as the Blue Men of teh Minch, given the color of their skin and their original habitat, live in underwater caves.
They create storms to make shops capsize, for seeing people drown is one of their favorite activities. To save yourself and your vessel your best hope is to distract them with riddles and rhymes, though this is not a simple endeavor for they will only pay attention to songs they don't know and riddles they have never heard before.
They may be the decedents of Moors who were enslaved by Norsemen going Viking and subsequently abandoned in the Minch during the 9th century. They differ from ordinary mermen in appearance in that they are totally blue and have grey beards.
-Matthews, John & Caitlin. The Element Encyclopedia of Magical Creatures. Barnes & Noble Publishing, Inc.: New York, 2005.
-Rose, Carol. Giants, Monsters & Dragons: An Encyclopedia of Folklore, Legend, and Myth. W. W. Norton & Company: New York, 2000.