If the hydroxide ion is such a poor leaving group then why does water dissociate? Water dissociates into the hydroxide and hydronium ions but, why does this happen if the hydroxide ion is such a poor leaving group. Alcohols don't undergo $\ce{S_{N}1}$ reactions because the hydroxide ion doesn't dissociate so why does it happen in water?
Only 1 in 10,000,000 water molecules dissociate. 

HI, HBr and HCl fully dissociate, and I-, Br-, and Cl- and are much better leaving groups.