A floating dam or island is provided by pre-fabricating modular hollow bodies. A first group of modular bodies is laid floating on a water surface, positioning the modular bodies in mutual side-to-side arrangement so as to delimit therebetween intermediate gaps within which the reinforcing rods are protruding. A first concrete casting is performed into the gaps and over the modular bodies so as to render them mutually joined. A second group of modular bodies is then laid over the first group and a second concrete casting is performed in order to join the first and second group together. Additional groups of modular bodies are laid and further concrete castings are performed up to obtaining a monolithic block having a desired floating dam or island configuration.