Borexino detector is composed of 2214 photomultipliers (PMTs) collecting the scintillation light produced by the liquid scintillator. Phototubes are partially immersed in the scintillator and partially in high purity water. A proper electrical and mechanical selection of the devices as well as a custom-made encapsulation was mandatory for a long-term immersion in these two aggressive liquid. Underwater cables and connectors and optical fibers are also described in detail.