I've never tried to sail in a sub, so I dont know if its realistic, but its very realistic in a normal ship to follow the waves. Actually I think the subs are not rocking at all compared to the wavesizes, but I guess that has to do with gameplay rather than realism. (I've been diving a lot in the pacifics, and sometimes the waves throw the (somewhat smaller) diveboat around like its a toothpick in a blender

Also some of the nightboats I've taken when I lived on Koh Tao (Thailand) was a fun (sometimes scary) experience, and those boats are around the size of a small sub (and keel-less).
Sometimes if you looked out the window you would only see the sea, then 5 sec later you would only see the sky. I guess since subs are keelless and slim, the wavesize and rocking would be even more drastic than is.
edit. and when the boat is in high waves, you wouldnt want to be on the deck. simply too dangerous.