A strong acid, usually sulfuric acid, H2SO4.
"Catalytic hydration" is just a fancy way of saying that ethene is reacting with water, with an acid catalyst.
The acid converts H2O to H3O+: H2SO4 + H2O <----> H3O+ + HSO4-. This is what actually reacts with ethene. The hydrogen bonds in water are too strong to break without the catalyst.
Protonating water increases its acidity, thereby making it more reactive.