When I made a water level, I added strong brewed tea to the water to improve the visibility of the readings.
I found that an easier method, requiring only one person, was to use my 4-foot carpenter's level. Three pieces of 2x2 lumber were cut (one 48" piece and two 8" pieces). An 8-inch "leg" piece was attached at right angles to each end of the 4' piece. The level was tied to the top of the 4' piece, so that it was raised above the ground while the legs at the ends of the level made contact with the ground. The leg ends could be pointed somewhat for more accuracy.
With a bag of pegs at my waist, I could quickly mark a contour line with that modified carpenter's level. It would take no more than 10 seconds to extend the line by 4' (at which rate I could cover about 25 feet per minute, working quickly by myself). The resulting contour line was marked every four feet.
Before using this setup, I checked that it would give the same reading when I switched the leg positions on the same two points on a hard surface (in other words turn the level 180 degrees and see if you get the same reading). Corrections could be made, if necessary, by lengthening one of the legs a little bit (easy to do if it's screwed together: just unscrew the leg a little until it's correct, then immobilize the joint with some tape or a wedge).
Different length carpenter's levels could be used, and combined with longer pieces of lumber, if desired (with longer lengths probably requiring two people).
