Unplug the water bed heater.
Soak up all water surrounding the mattress.
Press your thumbs and index fingers together to make a teardrop shape, then push your hands against the mattress corners until you make contact with the frame beneath. Continue along the seams until you see a small bead of water form on the surface of the mattress.
Cover the mattress with dry sheets or paper towels to further pinpoint the leak. Tuck paper towels around the edges and beneath the mattress edge and wait for a wet mark to appear. Press on the bed to release some water from the leak.
Circle the hole with a marker.