The action of soaking or washing yourself in water is a bath. You might take a bubble bath to relax before bed after a stressful day.
Your soak in soapy water is a bath, and the tub in which you're lying is also a bath. Other containers of water are baths too, particularly in cooking: "If the custard's sitting in a hot water bath, it will cook more evenly." While the Old English root bæð means "immersing in water," it also means "immersing in mud." If an investor "takes a bath," it means, colloquially, that she loses a lot of money.