Can water be a Lewis acid?
Strictly speaking, a Lewis acid is something that forms a bond by accepting an electron pair form another molecule (Lewis base).  Water as such does not do that, rather it is a hydrogen ion from the water that does so.  When ammonia acts as a proton acceptor in water, the Lewis theory calls it a displacement reaction where the acid, $\ce{H+}$, is initially combined with one base ($\ce{OH-}$) and ends up combined with another base ($\ce{NH3}$).