Thirteen new rotational transitions of H2O+ in the (0,0,0) level of the (X) over tilde B-2(1) State have been measured in the wavenumber region between 80 and 200 cm(-1) (50 and 120 mu m) by far-infrared laser magnetic resonance (LMR) spectroscopy. LMR data measured previously between 25 and 90 cm(-1) (110 and 400 mu m), as well as optical and infrared combination differences, have been combined with the new LMR data in a weighted least-squares analysis using an A-reduced expression of the rotational-fine structure Hamiltonian. Thirty-two molecular constants were simultaneously determined, some sextic centrifugal distortion parameters and some quartic and sextic spin-rotation parameters for the first time. From this improved set of molecular parameters, very accurate calculations of relational term values and zero-held predictions of the 1(11)-0(00) transition, including hyperfine structure, have been performed. Moreover, the electronic g-tensors and the hyperfine coupling constants are consistent with ab initio calculations which had been carried out for these constants.