Implementing the ab initio molecular orbital (MO) method, electronic excitation energies of Mg+(H2O)(n) clusters have been calculated in order to analyze the photo-dissociation excitation spectra reported by Fuke and his co-workers. Observed bands for n less than or equal to 2 are assigned to the s-->p transitions of the most stable isomer of Mg+(H2O)(n), with all waters directly bended to the metal ion. For n=3, n=4 and 5, the dominant bands are also those of the most stable isomer, [Mg+(H2O)(3)](H2O)(n-3), which has three water molecules in the first hydration shell. There are, however, shoulders and weak peaks in the experimental spectra, and calculations prove that these are due to the s-->p transitions of less stable isomers. The calculated spectra strongly indicate the coexistence of a few isomers for n greater than or equal to 3 in molecular beam experiments.