A bathing machine : a device, popular from the 18th century until the early 20th century, to allow people to change out of their usual clothes, change into swimwear, and wade in the ocean at beaches. Bathing machines were roofed and walled wooden carts rolled into the sea. Some had solid wooden walls, others canvas walls over a wooden frame, and commonly walls at the sides and curtained doors at each end. [i]. The style of machines evolved over the years. Images of various styles can be found as notes to individual songs.

Reference:

[i] https://en.wikipedia.org/wiki/Bathing_machine