I agree, water levels are extremely accurate. I made a simple one for laying out the footers for my garage.
You do not need to have alot of the clear tubing, the major cost. I bout 24" of clear tubing and two garden hose repair ends. The type with the barbed end, one male, one female. Cut the tube in half, jammed a repair end into each, then attached them to a garden hose. Filled up the hose, and worked all the air out. Total cost was about $4, I already had the garden hose.
I only had 12" of clear on each end, but I didnt need that wide of range. The only problem I had was the water level would bounce after I raised or lowered one end. Using a resevoir would eliminate that, I do have some 5 gallon buckets!
Keith in SC