Mac, I was hoping someone else could explain this to you, because I'm not really sure of the answer.

I think there were watertight doors above the waterline, such as on F Deck for example.

On Marconigraph.com, in the article: James Cameron’s Titanic Expedition 2001: What We Saw On and Inside the Wreck



"These lowest of the first-class public rooms, on F Deck, were, as I said, high on our list of priorities to explore. I had surveyed the portholes to the Swimming Bath from Mir 2 and found a few of them to be almost entirely clear of rusticles. If an ROV could get in there, we would pour as much additional light in through the unobscured ports as we could from Mir 2. Our access to this room, however, was in question as there is a watertight door between it and the first-class stairs and between it and the Turkish Bath, both of which we expected to be closed".

However, I do not think I have ever seen a picture of a watertight door other than the ones in the boiler rooms.