The term “boat bath” can encompass a range of things, from a bathtub shaped like a boat to a bath designed for boats. The meaning is typically clear from the context, although you may want to be specific in certain communities to avoid confusion.
In the first sense, a boat bath is a deep, free standing bath which typically has taps located in the middle of the tub, if at all. The design is clearly meant to mimic the baths used on board ships historically, which were designed to be filled with water and dumped over the side. Many boat baths are double ended, and they can look quite elegant in a range of bathrooms. A bathroom supply store is a good source for this sort of boat bath; you can also purchase restored antique boat baths from specialists.
These boat baths are available in a wide range of materials, including an assortment of metals. For an authentic look, some companies make boat baths from copper, which is left untreated so that it acquires a natural patina. It is also possible to find boat baths in acrylic, enameled metal, porcelain, and other materials. Given the range of materials and tap configurations available, if you want a boat bath, you may want to think about exactly what you need before you go shopping.
The term can also be used to describe a bathtub which is shaped like a boat. Although a boat bath might be a novelty, it is generally perfectly functional and sometimes quite fun to use. These sorts of boat baths are also typically free standing, and they may come with feet or cradles to elevate the bath above the floor. Stone, wood, and acrylic are also used to make boat baths in this style; since they are specialty items, be prepared to pay a premium price.
In the final sense, a boat bath can be a floating tub which is designed for the purpose of cleaning boats. It can be floated next to a dock, allowing the user to move a boat into it, and then filled with chemicals which scour the boat, removing algae and other forms of marine life. Boat baths are also sometimes called boat liners, and they are available from specialty boating suppliers.