Oceans cover more than 70% of the Earth's surface, with an average depth of approximately 3,800 meters. Humanity is largely excluded from this vast world of saltwater, which remains mostly unexplored and unknown. The goal of this book is to introduce readers to this unfamiliar and mysterious realm. It delves into various species and communities in great detail, emphasizing that the taxonomy, life cycles, and ecological dynamics differ significantly from what is typically taught in basic Biology and Natural Sciences courses, which often focus on terrestrial or freshwater environments, and occasionally, benthic habitats. The pelagic lifestyle, by contrast, is rarely emphasized in Marine Biology textbooks. This book aims to shift that perspective, highlighting the pelagic way of life for marine organisms. As a teaching method, species, geographical locations, and key topics are repeatedly revisited throughout the text to enhance their visibility and reinforce their understanding in the minds of students.