Insects are the most evolutionarily and ecologically successful group of living animals, being present in virtually all possible habitats on the mainland, but virtually absent in the ocean which constitute 90% of the biosphere. Only a few species are present in the sea, but remaining at the surface, salt marshes, estuaries, or shallow waters. Remarkably, a group of 13 species manages to endure long immersion periods in the open sea, as well as deep dives, i.e. seal lice. Sucking lice (Phthiraptera: Anoplura) are ectoparasites of mammals, living into the fur or among the hairs of their hosts. Among them, the family Echinophthiriidae is peculiar in the sense that it infests amphibious hosts, such as pinnipeds and otters, who can perform deep dives and remain with their bodies submerged from several weeks to several months. During the evolutionary transition of pinnipeds from land to the ocean, echinophthiriids lice had to manage the amphibian biology of their hosts, some of which may spend more than 80% of the time submerged and performing extreme dives at beyond 2000m under the surface. These obligated and permanent ectoparasites have adapted to tolerate hypoxia, high salinity, low temperature, and, in particular, to tolerate conditions of high hydrostatic pressure. We will discuss some of these adaptations allowing seal lice coping with the amphibious habits of their hosts and how they can help understanding why are insect so rare in the ocean.