heat must be given off by the system, i.e. the water, in order to slow everything down enough for the crystal to form.
As aresult, the freezing of water is an exothermic process because heat is being removed from the system.
An endothermic process would imply that heat must be supplied to the system. That is clearly not the case here, since providing heat would actually increase the average kinetic energy of the water molecules.