The peculiar propensity of water to have a high internal dynamic activity in its molecular complexes with organic molecules is described in this paper. Often, the corresponding large amplitude motions are reflected in the tunnelling splittings of the rotational transitions which, in turn, provide information for the determination of the potential energy surfaces and of the noncovalent interactions of water with a variety of atoms and/or functional groups. A classification of this kind of molecular complexes is given, also in relation to the tunnelling features of the rotational spectra. As a specific example, the rotational spectrum of tert-butylalcohol–water, investigated by Fourier transform microwave spectroscopy, is reported. Details are given of the large amplitude motions which take place in the adduct, the internal rotation of the hydroxyl group and the oscillations of the water molecule, by interpreting the experimental data with a flexible model.