Answer
The conjugate base of $H_2O$ is $HO^-$ (or $OH^-$).
Work Step by Step
To find the conjugate base of an acid, just remove one proton $(H^+)$ from the compound.
1. Remove the hydrogen:
$H_2O -- \gt HO$
2. Subtract one from the charge
$0 --\gt -1$
$HO -- \gt HO^-$
3. Normally, this ion is represented by $OH^-$, but you can also represent it by $HO^-$.