Yeah, back then we just had really big holes and cracks in the mountain. No huge flat expanses of open space like we have now. All the subterranean creatures would be concentrated in those little spaces, and opening one up was pretty dangerous.
I remember I once lost about a dozen dwarves when I cracked an underground river right above a waterfall. One of the dwarves went in there to fish or drink from it and got attacked by a giant toad or olm. The rest of them followed to pick up the socks.
I also had a giant cave swallow fly out and start pecking dwarves to death. Just going after one at a time, nobody coming to their rescue despite it being in the middle of my meeting area. Marks dwarves were ignoring it. Fucker eventually went back to the chasm and stayed there.
Also, the new caverns can be completely dry sometimes as well. Or at least that's happened in the past, like the 31.x versions. In that case theres also no vegetation. Its not common though, and that being the case in all three cavern layers is even more unlikely.
Now, to creating water... I think you could do it via reaction if the reaction gives a glob of basic water in a bucket as product. That requires modding, and since you've already started the game you'd be replacing a preexisting reaction. No other good ideas...