How to make a cheap & easy water level.
A water level works on the principle that when put in a tube, (imagine the tubing shapped in a large "U") the meniscuses on both ends of the tubing finds the same height.
Start w/ some 3/8" ID clear vinyl tubing, a sharpie marker, and a some water in a dixie cup. Get a lot of tubing.
There will be a fixed end to your tube and a working end. Mount one end (which will be the fixed end) of the tubing several feet higher than the reference height you want to work at. The reference height is the common height (elevation) from which you're going to measure everything up or down, just like you would do with a rotary laser level -- maybe the height of the top of your base cabinets for instance if your application is setting base cabinets. DO NOT CAP THE END -- it must be open to work properly. I'd just put a nail straight through the fixed end of the tubing and mount it to a 2x4, some drywall you can easily patch, or even tape it to something, if you can tape it very securely. Here's the important thing, the fixed end cannot be allowed to move up or down.
Then put some water in the tube, please don't fill it up. Fill it up from the "working end" of the tube. When the water fills the tube on the floor and the miniscus starts rising up the toward you, do this... WATCHING THE MENISCUS ON THE FIXED END, raise it to the exact height of your reference height, not by adding water... but by pulling tubing on the working end off the floor and above the reference height. Once you've done that, you have a giant "U" with both meniscus's at the reference height. If you still have too much dry tubing on the working end add some water and keep more "wet" tubing on the floor. Ideally you want a few feet of dry tubing when both meniscus's are at the reference height. Then use the sharpie to mark the tube at the meniscus on the WORKING end.
At this point, your level is calibrated and there is one rule: Since you can't plug the tubing at either end (it won't work)... DON'T DROP or lower the working end to the point that the water drains out! wrap the working end of the tubing a few inches below the opening with some duct tape and punch a hole in it so that it can be hung high above the reference point off a nail, screw, or whatever.
Now you can take the working end anywhere - keeping the sharpie mark above the working end's meniscus and slowly lower the mark until it meets the meniscus. When they meet, you're at your reference elevation, exactly level with the reference elevation set at your fixed end.
I know it sounds like a lot of work, but if it sounds confusing, follow these instructions one time and it'll click. It'll all make sense and you'll have a great little water level that'll always works, inside, outside, over long distances, etc.
Any old-timer tradesman will know this technique and tell you it works. The really stubborn ones won't buy laser levels at all because they don't understand why they'd spend $1000 to do what in their minds $5 worth of tubing does.
Hope it helps someone.
David