Okay, so I built my fort under an ocean. After getting all the important bits set up and running smoothly, I decided, "I need a waterfall in my main hall!" I dug a tube through my main hall down to the first cavern, put a floor grate on it, and then built an upramp into the ceiling right under the ocean. I saved right before just in case of massive flood. However, it worked perfectly; the water flowed straight down from the ocean, landing in the cavern, and created tons of mist. I continued to play for a few more seasons with no flooding, then saved the game and went to bed.
I load up my game in the morning, and immediately notice that there is water splashed all around the edges of each of the channeled holes near my waterfall, and spreading quickly. The ocean, which had been a big huge swirling mass of 5's, 6's, and 7's had become a solid mass of 7's again.
I'm maybe 95% sure I had no flooding before I saved last night. I'm pretty much 100% sure the ocean was only getting MORE drained as well, not less drained over time. And yet I reload to find it completely refilled and flooding.
Is this a known bug that infinite water sources instantly refill upon reloads? It is yet to be apparent to me whether this sudden, massive influx or water will really flood my fortress or not - it's slowly filling up my dining room, but only very, very slowly - but regardless of whether or not it'll spell the end of my fortress it's going to make it very messy, which is just sort of annoying.
Of course, this could all have been avoided had I built in a stop-gap measure in case of floods. But that's sort of beside the point. The point being, is this a real bug?