The magnitized needle works without the foam as well. It will float just using surface tension on the water.
The water finding technique is commonly called "witching a well". I do a version of it before I begin digging a new swimming pool in someone's back yard. The Y shaped stick has to be a fresh sap filled twig to work. My grandfather prefered to use any of the fruit tree varieties. I find that a pair of L shaped wires works better for me. I can do it with reformed wire coathangers or wire survey flags, but heavy guage copper wire is more accurate. The 4" or so leg of the 14" L goes loosely through the fist, hands held in front of you parallel with thumbs up. The wires rest on the curled index finger and pivot around the vertical short leg in your fist. Hold the fists barely against each other and begin with them slightly tilted so both wires point perpendicular to your body straight out in front of you, parrallel to each other. Walk slowly across the area you want to survey. When you cross an object which has electrical charge, the wands will begin to swing toward each other. When you are directly over the object, the wands will cross completely, the left pointing right, and the right pointing left. Mark the spot and continue walking. Recross the marked area from 90 degrees to see how the object lies. I use this to locate septic tanks, sewer, water, gas, electrical, and phone lines. It is crude, but it does work within reason, and is nearly as accurate as the magnatometers the utility guys use. Give it a try, you might be surprised.
Codger