New to the forums and found that this thread seemed to focus most on one of the topics I would like to see implemented. I would be interested in taking over for you Barbarossa, but I need to be certain I can devote the requisite amount of time to the forums first. We'll see what I can accomplish in a month.
As far comments go. I would like to see the underwater world as functional as the overworld. To this end I think the priority should be:
I-Z-level pathing
First and foremost for interacting with an underwater world is navigation. There's not much to do underwater until dwarves/aquatic creatures can navigate properly. Also helps with flying creatures like eagles, dragons, etc.
II-Mounts and carts
Once dwarves get to swimming, it will be useful to be able to use boats and the like for transit of goods. Allowing players to use carts would be a step in this direction (I think, again new here). Of course, if larger ships are to be implemented as buildings as previously suggested in this thread, we'll need...
III-Structures that exist without support
As far as I know at current, any structure that is without support is broken down into its base materials before traveling through the air. Similarly any building unsupported in the water will do the same. This would probably have something to do with buoyancy. Additionally it would allow interesting interactions on land. Assuming structures remain stable while falling, would the entire structure break on impact? How fast does it fall? Has applications for Icebergs and seasonal ice as well. An implementation of pressure for structural support could lead to things like...
IV - Atmosphere
This might already be in-game, don't know for sure. I don't know if it's realistic either. But the map could in theory be "flooded" with a fluid atmosphere, and possibly use existing suction mechanics to move it around. This would allow for cooler weather effects (like waterspouts) and more subtle ones as well like a simple breeze. I suggested it here since it seems related to water. Will suggest on the aboveground diversity thread if the necromancy isn't frowned upon.
I feel most of the fun from dwarf fortress is that on the surface it realistically simulates the world, but in depth there are funny little quirks that are difficult, destructive and Fun to manipulate. I feel like most of these suggestions add more of this to the game.