Explain the nature of boric acid as a Lewis acid in water.

Orthoboric acid is less soluble in cold water but highly soluble in hot water. It is a monobasic acid. It does not liberate H+ ion but accepts OH- from water, behaving as a Lewis acid.
${\mathrm{H}}_{3}{\mathrm{BO}}_{3}+{\mathrm{H}}_{2}\mathrm{O}\to \mathrm{B}{\left(\mathrm{OH}\right)}_{4}^{-}+{\mathrm{H}}^{+}$
The structure of H3BO3 is
Octet of boron remains incomplete. Oxygen atom contains lone pair of electrons in water molecule. Hence, instead of donating proton (H+), boric acid accepts OH- from water forming $\mathrm{B}{\left(\mathrm{OH}\right)}_{4}^{-}$ to complete octet.
Since, electron acceptor substance behaves as Lewis acid, therefore, boric acid acts as a Lewis acid in water.