How light would I have to be to walk on water?

The surface tension of water at room temperature is around 72.7 millinewtons per meter, or 7.42 grams-force per meter.

The force supporting you on the water will be roughly the surface tension times the circumference of your footprint on the water. You can weigh 74.2 milligrams for every centimeter of that circumference. The bigger your footprint the better.