What does dowsing mean?

Dowsing is a type of pseudoscientific divination employed in attempts to locate ground water, buried metals or ores, gemstones, oil, gravesites, malign ‘earth vibrations’ and many other objects and materials without the use of a scientific apparatus.

Is it douse or dowse?

You douse a fire with water; you dowse for water with a dowsing rod. Unless you are discussing the latter practice, the word you want is “douse.”