With ab initio molecular orbital calculations, structures of the singly positive charged boron-water clusters B+(H2O)(n) and aluminum-water clusters Al+(H2O)(n) are determined. The insertion reaction products HBOH+(H2O)(n-1) and HAlOH+(H2O)(n-1) are also investigated. Structures of the dimer-core clusters [M(+)(H2O)(2)](H2O)(n-2) are similar to each other for M = B and Al. In contrast, the stability and structures of [B+(H2O)](H2O)(n-1) and [Al+(H2O)](H2O)(n-1) are quite different. The monomer-core boron clusters [B+(H2O)](H2O)(n-1) do not have stable local minima; the spontaneous proton transfer reaction takes place to form BOH(H3O)(+)(H2O)(n-2). In other words, the acid-base reaction takes place in such small clusters. In the larger clusters, the reaction further proceeds and the clusters isomerize to the linear type HBOH+(H2O)(n-1). On the other hand, in the clusters [Al+(H2O)](H2O)(n-1), no such reaction takes place. The cis-form hydrated products of the insertion reaction HMOH(+)(H2O)(n-1) are more reactive, and the acid-base reaction is seen for both M = B and Al.