Lichens are a symbiotic complex of autotrophic (algae, Cyanobacteria) and heterotrophic (fungi) components that have developed during evolution in coastal ecosystems in the process of adaptation of algae and fungi to terrestrial habitats. Lichens are highly adapted to extreme habitats including the littoral (or intertidal) zones of coasts. In this chapter, we present developmental stages of aquatic lichen investigations: freshwater and marine lichens. The issues of species diversity of coastal lichens, their ecology, and adaptations to the coastal marine environment are described. The leading factors affected the epilithic lichen cover of coasts, and freshwater habitats are at a distance from the waterline and substrate characteristics. Substrate characteristics, especially near the waterline, depend on the wave rhythm. On the coasts of freshwater bodies, four zones are recognized based on flooding duration and lichen ecology. Lichen zones of fresh and marine coasts are distinguished by their species composition: on sea coasts halophytes are predominant and on freshwater shoreshydrophilic lichens. Marker species of lichens were identified for each zone. For the littoral zone, the intrazonal structure of lichen flora was shown. In the adaptation of symbiotic organisms, such as lichens, both symbionts take part: mycobiont and photobiont. Morphological and structural adaptations are mainly associated with mycobiont variability: the presence