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.
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 B(OH)4- to complete octet.
Since, electron acceptor substance behaves as Lewis acid, therefore, boric acid acts as a Lewis acid in water.