Water monomers and dimers have been studied at low temperatures in matrices of solid p-H(2), o-D(2), n-H(2), and n-D(2) using infrared spectroscopy. Our data demonstrate interaction mechanisms between host matrix and guest molecules that are different from the ones observed in atomic noble gas matrices. Notably both guest/host rotational--rotational interaction and matrix induced modifications of the guest libration modes are observed. We also show that different types of interaction influence the relaxation times of some of guest modes. Water rotates freely in p-H(2) and o-D(2) but librates in n-H(2) and n-D(2). Rotational relaxation is faster in o-D(2) than in p-H(2) and faster in p-H(2) than in Ne. This is attributed to interactions between water rotation and matrix molecule rotation in p-H(2) and o-D(2). In n-H(2) and n-D(2), a strong water libration band is observed in the far-infrared, and strong water monomer vibration bands have libration satellites. Water dimer bands, close to matrix rotation bands, are perturbed by the matrix motions. The H-bonded isomer H(2)O--HOD rapidly converts to the D-bonded form H(2)O-DOH in p-H(2) and in o-D(2) but slowly in n-H(2) and n-D(2).