Question

Why can water be a Lewis base?

Answer 1

Answer
A Lewis base is an electron pair donor. Water has one active lone pair that can do so (forget about having four-coordinate oxygen).
In this reaction, water is the Lewis base, $H^{+}$ is the Lewis acid(not$N{H}_4^{+}$) Water donates the electron pair, and the $H^{+}$ on $N{H}_4^{+}$accepts that electron pair to make a bond.
$N{H}_4^{+}$is then a Brønsted-Lowry acid, being a proton donor, and water is then also the Brønsted-Lowry base, being a proton acceptor.
The way I drew hydronium is a bit misleading. What is the actual molecular geometry of hydronium?