Answer
The conjugate base of $H_3O^+$ is $H_2O$.
Work Step by Step
To find the conjugate base of an acid, just remove one proton $(H^+)$ from the compound.
1. Remove one hydrogen:
$H_3O^+ -- \gt H_2O^{+}$
2. Subtract one from the charge
$+1 --\gt 0$
$H_3O^+ -- \gt H_2O$