solving equation with pde tool

hi, im am trying to solve this equation on a 2D domain: ni = H/2*cos(k*x-sig*t), water surface equation.

i've created the geometry matrix and the boundry condition matrix (edges on which the ni,H,sig values are known).

i'm having difficulties to understand how to incorporate all of that into the PDE toolbox or its functions in order to solve this equation.

i'm grateul for any type of help. thank you.