In most cases, you will use an acid catalyst in the formation of imines from amines and ketones. However, it generally isn't done in the presence of aqueous acid (H3O+). The reaction is reversible, and since water is a prodcut of imine formation, the presence of water in the reaction will shift the equilibrium to favor the reactants (in fact, aqueous acid is used to hydrolyze imines back to amines and ketones). A lot of the time, I see glacial acetic acid used as a solvent for imine formation since it is an anhydrous acid.