Divers' milieus refer to the different underwater environments that divers explore. These environments can range from shallow coral reefs to deep ocean trenches, and each environment has its own unique set of flora and fauna. The term 'seabeds' is used to describe the bottom of the ocean or sea, which is where divers often explore. Seabeds are composed of a variety of materials, including sand, mud, rocks, and coral, and they provide a habitat for many species of marine life.