This issue of Photonics Focus explores the world of fluids and their interactions with optics and photonics, including how photonics is helping us find microplastics in the world's oceans, and how innovative optics designs enabled the CIA to find a sunken Soviet submarine for its daring Project Azorian. We also look at the undersea world of Dimitri Rebikoff whose inventions like the first waterproof flash enabled an Oscar-winning film by explorer Jacques Cousteau.