Tomorrowland’s lagoon always captures the attention of passing guests. The deep blue waters, squawking seagulls, and circling submarines stand out from the overwhelmingly large buildings nearby. When the lagoon opened in 1959, the park also debuted a brand new attraction: the Submarine Voyage. Though the vast majority of this attraction took place underwater, Disney added an element to draw in visitors from the shore: Lagoon Mermaids!
For four summers between 1959 and 1967, mermaids hung out in the lagoon alongside the passing submarines. They mostly lounged on the rocks, though they occasionally dove into the water and swam past exploring submarines. As a result, mermaid cast members had to be strong swimmers, and were trained to stay far enough away from the subs to avoid any accidents.
Source: Pop Sugar
Unfortunately, the combination of highly chlorinated water and the risk of injury meant a short lifespan for the mermaids. The last mermaids swam in the lagoon at the end of the summer of 1967. For many years, the lagoon had little above water to draw guest interest. But when the attraction became the Finding Nemo Submarine Voyage, seagulls landed on the buoys, chirping their famous line from the film: “MINE!”