Below what depth would a submarine have to submerge so that it would not be swayed by surface waves with a wavelength of 12 meters?

You can't base surface wave action ability to affect a submarine based on wavelength or even height alone. It depends entirely on the dynamics of the surface wave action as it's affected by the weather, the size/type of the submarine involved, and a slew of other ocean environment variables. A smaller boat will be affected by wave action at depth more easily than a larger one will.

Of course I'm sure there's some classroom Oceanographer/Professor/High School teacher with no real-world submarine experience somewhere who thinks it's possible to calculate it without minor variables like submerged displacement, propulsion type and speed (DE or Nuclear), depth of the boat in question, density of the water (e.g., in or out of the Gulf Stream, or in the Arctic, where salinity is less), temperature, wind speed, storm action, etc.

Intense storms can have a significant effect on submerged submarines at even 400' and below. I remember in particular a remnant of a hurricane as we were headed home to Charleston in 1983. We were at 400', taking 12-15 degree rolls due to the intense wave action on the surface. Mother Nature is not friendly when she's angry.