The submarine would not experience much of anything. The amplitude of a tsunami wave is rather small in the open ocean. It becomes large only where the depth of water is low as it is along a beach.

The submarine would briefly experience very powerful currents driven by the cycling of the ocean. It might get carried to depths where its hull would not be able to resist the pressure and the ship would implode, but otherwise it would just get sucked around.

Of course if the submarine were too close to shore where the waves are breaking, then the sub would get smashed just like everything else.