What is missing is the density of salt water. Let's denote it ρ.
The pressure at depth d then is:
p = p0 + ρ g d
, where p0 is the atmospheric pressure at the surface of the sea. We assume it is equal to the 1 atm inside the sub.
Hence the pressure difference between inside and outside of the window is:
Δp = ρ g d
The force on the window is this pressure difference, multiplied by the area of the circular window of diameter D. The latter equals
A = ¼ π D².
So
F = p A = ¼ π D ²ρ g d.
This should be less than Fmax.
Hence d < Fmax / (¼ π D ²ρ g ).