I figured that I'd start the "Nautical Questions" series of the Steampunk Shipyard with one I encounter in non-Steampunk circles, that is "What is it like being underway"? To best answer that, it really does depend on the weather, or the sea state. Sea States dictate how calm (or not) the seas are, and are rated "1" through "9". The higher the sea state, the more pitching and rolling one experiences on a vessel. So if there is a large storm above the water, any submerged vessel below it will feel the effects of it. The deeper a boat is, the less the sea state will affect it. So, if you want to avoid the effects of a storm, a ship's Captain will (or the Officer of the Deck) have the ship dive deeper.
Now to answer the question, "what is it like under water"? Sad to say, with a lower sea state (1-3 on the Douglas scale), which is usually the case, its like being in a building with florescent lights. Not much "rocking or rolling", but its pretty level. On occasions where evolutions take a ship to the surface, there may be some rocking, but generally, its pretty level.
Steampunk Note - Unless a ship is diving or conducting an "emergency blow", the crew won't notice a difference on their daily life. There are exceptions, but its essentially "full steam ahead" for the boat!