On the rolling deep, when there's nothing to rest your eyes upon save the horizon, you get lulled into a false sense of security which can easily spell your doom. For no man knows the day or the hour, nor does any sailor know when a small dot at the edge of his vision will turn into a Nilfgaardian dromond racing towards him, its sails puffed full and its prow ready to cleave his hull like an axe.
Prevention is the first and best way to avoid such collisions: you must keep your eyes pried wide open, even when the waters are smooth as a newborn's hinterquarters and there's not a speck of land nor a hint of foreign flag to be seen. When vision fails in dark or murk, one must keep an ear cocked, especially in the waters near Skellige, for the oarsmen of the Archipelago add muster to their rowing with song and their thunderous voices carry far, so they can often be heard before they are seen. And it's a powerful boon that they thus reveal themselves, for a collision with a Skellige longship damns one doubly, for the vessel itself is armored prow to stern, while inside are ferocious, battle-hardened warriors.