Just a little thought: Maybe add a line in the watch officer's dialog box while on surface to request sea men to clear out the deck, just like they do when ordered to submerge. Although it looks nice on very calm weather, it feels odd past any forms of waves, even small one (Especially since clothes have not been yet modded out). This way, you won't have to add any icon to the GUI and won't mess with people's mod choice regarding this. Otherwise, it looks very nice when submerged and adds to the immersion factor!
Maybe when surfaced would you be able to make some seamen remotely go back to the inside, some seamen wandering about the sub back and forth from time to time, to add a little more life to it. I guess it is planned, though...
