It's a cool idea, but I'm not sure how workable it would be with sky ways as they are now. I think if I had one I'd want to have an underwater area for it to be used. That would fit in with your idea of it being for new and high level worlds. You could use it in the regular areas like a regular ship ( sailing like a surfaced sub). As all the ships are in the air what they look like doesn't make a huge difference. Boarding actions could take place through a couple airlocks. In a new area, there could be a Celestia like water zone. In order to enter those areas, a submarine would be needed. I see there being a starter area with some regular land and sky ways quests you do in order to obtain your first sub, or buy one (kind of like Skull Island started us off). Then the next area would have a sky way transition water wall and ships that warn you you need a submarine to go further. Instead of the sky ways, there would be currents of water in those areas and a bit more of up and down travel.

I'd like to see them add more raft types or perhaps some sloops into the game before getting into submarines. A class of small ship that moves quickly, mostly for traveling and collecting missions. I'd also like access to those steamboat ships as a purchasable option. Once the do those, sure, Submarines would be cool. I could see you getting one in Polaris, since nothing in the story has ruled out them having such a thing. What would be really good about Submarines? You could add another layer of missions and enemies to every single area. Skull Island, for instance, could have high level foes lurking deep beneath the clouds, waiting to be discovered and vanquished. New places to explore in areas we already have unlocked. A submarine would be great for battling large skyway monsters... (Giant Squid, anyone?) Like you said, no sails, instead the ship would have turbines. Torpedos or rockets instead of cannons. (Perhaps you'd actually have to aim them, only shooting straight foward? Then you could make them very powerful, since they could be flanked easily.) I'd say when submerged you'd have airlock combat, instead of on deck combat... but whatever works.

But, since you brought it up, if you want to see submarines, wouldn't it be cool to see High Altitude ships? Make Zeppelin's for each nation. You could have similer results to the submarine... new areas, new foes all in current locations. Alternatively, you could combine Zeppelin's and Submarines. Higher or lower pressure, you still need special systems to maintain hull integrity, and seal the atmosphere. Since it's all sky, you could engineer the system to go both ways.

On a similer note, our characters should have deep sea diving gear when underwater, since there is no timer on how long we can hold our breath. It would make more sense. I only know of one mission where it matters, so far... but still.

The ideal is quite fancy however, as said it would be hard to explain to those non Marleybonian(Celestian StomrRiver submarine) or Valencian(Marleybone's ideals are based off of Valencian) . And unless you're a modern pirate I don't think you'd need such a contraption... However, this does remind me of an ideal I had conjured(unless someone thought of it first, I apologize) but, what if we could go inside the ships cabins like the ship in Wizard101's Azteca? We could decorate it and call it our second home for on the go...it would also create another activity and new way to decorate while waiting for other updates to arrive...just a though.

-sincerely, the Theurgist-Privateer.