For reactions in aqueous solution, such as an acid dissociation reaction 
:AH + HO  A + HO 
the concentration of water may be taken as being  constant and the formation of the hydronium ion is implicit. 
:AH  A + H
Water concentration is omitted from expressions defining equilibrium constants, except when solutions are very concentrated.
: (K defined as a dissociation constant)
Similar considerations apply to  metal ion hydrolysis reactions.