Magnesium chloride is readily soluble in water so there is no real reaction.
[check in solubility table at http://en.wikipedia.org/wiki/Solubility_...
If you want to represent it by an equation, then
MgCl2(s) + H2O ---> Mg2+(aq) + 2Cl-(aq)
will serve the purpose.
It certainly doesn't react to form insoluble white magnesium hydroxide, as suggested in another answer!
Source:
retired chem examiner