Answer
The conjugate acid of $H_2O$ is:
$H_3O^+$
Work Step by Step
To find the conjugate acid of a base, you just need to add a proton $(H^+)$ to the compound.
1. Add one hydrogen.
$H_2O -- \gt H_3O$
2. Add "+1" to the charge of the compound:
$0 -- \gt +1$
$H_3O --\gt H_3O^+$