Refers to the bathtub itself, often in the context of a Japanese-style bath. Commonly used in phrases like 湯船につかる (soak in the bathtub).
I fill the bathtub with hot water.
I'm tired, so I want to soak in the bathtub for a while.
rental boat with a bathtub (Edo period)
Historical term for a boat equipped with a bathtub that was rented to customers during the Edo period. Not used in modern Japanese outside historical contexts.
In the Edo period, there were rental boats called yubune.