Plus there is the idea that Bouyancy isn't anti-swimming skill. It is anti-floating skill.
In theory even a Bronze collosus could "Swim" by "Walking on the bottom of the water" (actually the Bronze collosus I wonder if he would be affected at all by the water)
Not sure I follow. If you're denser than water you use Swimming to swim up, if you're lighter than water you use Swimming to go down. What's this Bouyancy skill?
Also I figure a bronze colossus would literally walk on the bottom of the water, he doesnt need to breathe afaik so whatever.
Edit: Can't this be done simply by calculating average density at all times? If an item is concave it is partly "made up of" air which lowers its density. Whether or not that empty pocket of air will be replaced with water seems like a matter of chance entirely. A dwarf wearing nothing has a lower density than a dwarf wearing steel plate, floats/sinks accordingly. Has the added benefit of you being able to dress your dwarves in corkwood "life-savers" that reduce their average density and make them float.
There's no clear cut answer for which tile an item should occupy.. but assuming a flooded fort should move items inside it the items would have to stay in a tile with water and only float up to the above tile if it has at least 1/7 water, wouldn't they?
Considering most rooms (I assume) end up being 1 tile in height, a gap in the roof would "catch" items otherwise making them fall perpetually, right? 7/7, floats up to 0/7, nothing there, falls down, floats up, falls down.. whereas if the tile is 1/7 it would be "floating" in that tile for the duration.
I suppose in this case a dwarf who is "floating" in a 7/7 tile with air above it would need special treatment not to drown.