Skin diseases caused by aquatic organisms are continually on the increase owing to the great number of people who now converge on the hydrosphere for holidays, sports and professional activities. Aquatic skin diseases are no longer only a seasonal affliction but can be observed at any period, thanks to the tourist boom promoting aquatic holidaymaking throughout the year. This book aims to contribute to the knowledge of some aquatic animals that have only developed a poisonous apparatus to protect themselves and adapt to their environment, as well as of the various clinical pictures induced by different pathogenic mechanisms. Besides the afflictions caused by biotoxins, skin reactions to microscopic organisms present in seawater, freshwater, swimming pools and aquariums are described, together with some non-biotic forms induced by direct contact with salt and freshwater. A better knowledge of the aquatic environment may help people to enjoy this enthralling habitat at lesser risk.