Looked through the first post's list, I didn't see anything about swimming, Dorf diving, or anything like that. It seems like a lot of the stuff mentioned would only be accessible via aquatic critters - treasure ships, and trenches, for one.
Dorf submarines and stuff would be fun and the easiest approach, I suppose, but I'm pretty sure that'd clash with that year-1000 technology thing. Making it easier to learn swimming would be a start - since next release has that class/mentoring thing, that'll be easier, I suppose.
I've actually
really wanted some method of getting dwarves to be able to function in the water. Like a diving suit, connected to a surface vent with a reinforced hose or something. That would be so rad. Semi-aquatic forts, with some pathways and sections flooded out would be awesome.
I'm also imagining totally undersea forts, like the habitats in X-Com: TFTD, with bulkheads and airlocks. Entering would require a pumping system to unflood the room before the inner doors opened, though I guess that could be circumvented with a pit under the entrance and a lot of grates. And pumps at the bottom of the pit. Also building destroyers! I guess that's also completely out of place along with subs, though.
Aquatic civilization cities would be cool - there wouldn't be a need for ground access, I would think, so there could be doorways anywhere. It would be towers instead of houses and shacks, like.
Also, HFS ocean trenches sounds amazing. I am a bit curious, though, what happens when you put a
under the ocean?