Wonderful World of Physics: Water Levels
During the planning process, Mr. B and I watched a movie where the builders used a water level to cut all their posts. We had never heard of such a thing, but after many projects trying to determine level across a wide area (fences, rooms, etc.), we decided that a water level was the way to go for our project.
The basics: water levels work because water seeks its own level. It has been a long time since I took a physics class and I don’t want to embarrass myself here, so take my word for it. A water level is just a long piece of tubing full of water. As long as both ends of the tube are open, the water will reach a level point on both sides. This is the same reason your basic level (with the bubble) works.
There are two types of water levels and two ways to use them.
1) Piece of tubing
Simple and straight forward. You fill a piece of vinyl tubing with water, both ends open. The water level in both sides in equal
2) Reservoir/bucket and hose
In this system a reservoir is filled with water and a tube runs from the reservoir. The water level in the tube is equal to the water level in the bucket.
The two ways to use your water level (which are really sort of the same) are to either mark everything with the desired finished height, or to mark everything with a level reference height and measure off of that. The bucket level will only really work for the second method unless you can get the bucket water level up to your finished height.
While everything on the internet says that this method is fast, cheap, and easy, Mr. B and I are having some issues with it, namely that it is slow. We are using about 100 feet of 1/2″ diameter tubing that takes a long time to stabilize at the right height. We also keep loosing water, which quickly throws everything off. If you are considering using this method I would suggest using larger tubing, or potentially buying one of the nice water levels with larger ends and good caps (like this one or this one).
I tried to take some pictures last night but they didn’t really capture the method, so I’m including links to several articles/videos for your edification.
Home in the Straw 
You could have used mine. MR. X
I have a 1-acre pond that fills up about 1/3 potential. After rainfalls, when it fills quickly, it always recedes to the same level, which logically would indicate the level or general location of the leak. We have used liners to cover much of the pond, with some success. However, is it also possible that the leak could be at a lower, not-visible level with water ‘siphoning’ out to a cavern/sinkhole/outlet?