Water piston?
If you dig out the bottom underneath the drowning chamber and have grates to allow the water to teleport in, dropping a block which is say, 5 z tall by however wide x/y into a 3 z deep chamber of water will give 1 z on top of the level where the block was and 1 z in the level above.
I've not done it with water which may behave differently due to pressure being a thing, I wouldn't be surprised at all if it blasts all of the grates into the air or something crazy like that.
But the piston needs to move down, where it displaces the water (gets teleported to the top), and the maximum of water "levels" you are getting on top is the number of z-levels you dropped the piston (there is some spill, the water needs to be caught with bridges on the sides, ...). If you have the drowning chamber at z=0, then you need empty space at z=-1. The piston drops, say, 2z levels into a full cistern. The piston top is now at z=-3. There is water all the way up to z=-1, but it won't be pushed up into z=0.
So unless you drop the piston
through the drowning chamber... (so cave in + drowning)
Edit: well, that is just what you suggested (on a second read...). Remark: Water teleports, there is no need for grates. There have to be bridges at some z-level below to retain the water... sadly this is a non-repeatable design (but could be made repeatable by pumping magma up and obsidianizing the top of the piston again).
Edit2: Looking at the maze: the hallways need to be 3 tiles wide at least in order to have the piton walls completely detached, so it can cave in properly ('cause the piston top needs to be the same level as teh drowning chamber). In the end this will just be a cave in trap, though - but with added drowning, and then magma do make it repeatable, which sounds quite dwarven to me.