Still fiddling around with a dungeon to sink under transparent overlays, but here is how I would keep track of the levels. Basically just a less granular rendering of the mire-o-meter I blogged about before.
I would describe these to players as 1) thigh-high 2) chest-high 3) over your head 4) you can't feel the bottom.
You could use smaller increments, but the larger ones were meant to limit the number of overlays I would need and because you probably won't see much change in dungeon features with just a foot of liquid added.
ps trivia: the silhouette is based off a pic of Eugen Sandow