Depending on what materials your harmonica is made from, you should choose one way or another when you want to wash it. It is recommended to run some basic maintenance on your instrument as a general rule. A model made with a plastic, or sealed wood body can be washed with lukewarm water.
Always tap your harmonica after playing to eliminate the saliva that gets inside. It is a good idea to take care of your oral hygiene, too, to limit the contaminants that can reach inside. Heavy duty cleaning that involves removing the plates can also be performed, but with extra care.