I'll take a shot.

Depending on your water level fluctuations, you only need to place a stake/level indicator in water just deep enough to be within the range of fluctuation (right?).

So if you water level does not vary more that +/- 3 feet, putting a stake in 4' of water should work.

After placing the stake (that already has tic marks without actual numbers), you could take a string with a heavy weight and plumb to get the exact current depth at the location of the stake.

Then go from there?

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."