(likely the first lifeguard post in Copacabana, in 1918)
In 1914, the first activities related to aquatic rescue began to take shape, when Comodore Wilbert Longfellow founded the first rescue service of the American Red Cross in Rio. The idea was to organize and train volunteer lifeguards who would operate from lifeguard posts all over Brazil.  While it actually failed to take hold initially, by the end of the 1910s, the growing success of sea baths provoked the emergence of lifeguard posts in Copacabana . But since it was only in 1939 that 18 fixed towers were built along the city’s coastline, the first posts (actual posts, as you can see) were nothing more than what can be observed in the images above and below.
Researching the development of these posts and towers, I couldn’t locate multiple, reliable sources about the exact structural progression (but this here is likely accurate) due to different sites showing different information as to exact dates but, if I had to take an educated guess, the second iteration of the lifeguard post in Rio is what can be seen below. The third photo, showing the lifeguard doing a backflip off the post, is dated 1929, and matches the construction of the post in the second photo. Seeing that photo 2 & 3 show the famous sidewalk, and knowing those Portuguese stones were put in place in the early 1920s, it’s safe to say the two photos below date from the period between the early 1920s and 1939, when the first actual towers were installed.
(source of image above)