Answer
$H_2O$ can act as both a Lewis acid and base.
Work Step by Step
If we draw the Lewis structure for the $H_2O$ molecule, we will see 2 lone pairs on the central atom $(O)$, therefore, it can act as a Lewis base.
To understand why $H_2O$ is a Lewis acid too, it's more complicated.
When $H_2O$ acts as a base:
$PH_3(aq) + H_2O(l) \lt -- \gt P{H_4}^+(aq) + OH^-(aq)$;
If we draw the Lewis structure for $PH_3$, we will note a lone pair on the central atom $(P)$
In the reaction, one of the hydrogens from the water molecule accepted the electrons from the "$PH_3$".
So, we consider $H_2O$ a Lewis acid in this case.
