Assuming operations below the levels where wind and waves are a factor. Does changing between sea layers, currents, or some other phenomenon ever result in the crew experiencing anything other then “smooth sailing”?
I served on the nuclear submarine USS Spadefish (SSN-668). At normal operating depths even a large storm on the surface would not affect the submarine. The Navy FAQ on submarines says that violent storms may be felt as deep as 400 feet Navy Submarine FAQ, see item 21. The deepest we ever felt surface effects was about 150 feet and it was pretty good sized storm on the surface above us.