The approach of two surfaces in a poorly wetting liquid (finite contact angles) is shown to lead to a decrease of the density of the liquid in the gap due to enhancement of thermal fluctuations in the lateral direction. A simple treatment based on classical nucleation theory shows that the decompression effect determined by the boundary conditions and the molecular discontinuity of the medium is essentially long range. The phenomenon accounts for the experimentally observed magnitude and range of the hydrophobic attraction and other deviations from Lifshitz theory, as well as for the much shorter range of the repulsive hydrophilic hydration force. The triggering of the basic mechanisms of phase transition via critical cavitation either at the interfaces of thick liquid films or across the gap at small molecular distances between solid surfaces is substantially dependent on the wetting hysteresis. The boundary decompression-compression transition caused by surfactant adsorption is important in preventing rupture of thin soap films. The hydrophobic effects of nonpolar solutes are briefly discussed.