Given the balanced equation representing a reaction: HCl + H2O H3O+ + Cl-
The water molecule acts as a base because it

It accepted a proton from HCl

Explanation:

When properly written, the equation box the reaction is given as; HCl(aq) + H2O(l) > H3O+(aq) + Cl-(aq).

According to Brownstead-Lowry definition of acids and bases, an acid donates protons while a base accepts protons.

Water molecule acts as a base in the reaction because it accepted a proton from HCl in the reaction above.