THE STORY Varga (Fernando Wagner), a villainous white pearl thief posing as the god Balu has chooses a lovely young native girl, Mara (Linda Christian) to be his bride. She escapes the island of Aquatania and meets Tarzan (Johnny Weissmuller) and Jane (Brenda Joyce) who agree to help her and she gives Jane a rare black pearl. She is recaptured, however, and taken back to the island. Tarzan goes to her rescue. Tiko (Gustavo Rojo) to whom Mara is betrothed meets Jane. Benji (John Laurenz), the singing mailman takes Jane's pearl to the region's commissioner, who decides to visit the island, picking up Jane and Tiko along the way. They are captured and Varga's henchman, Palanth (George Zucco), in his role as high priest, is about to sentence them to death when Tarzan, disguised as Balu, arrives to have them released. Tiko and Mara are about to be married when the real Balu appears and all the outsiders are ordered to be thrown to their deaths into the sea. Tarzan unmasks the false god and the two villains meet the fate they had tried to impose upon the others. 

Producer: Sol Lesser
Director: Robert Florey
Writer: Edgar Rice Burroughs (characters)
Writer: Carroll Young  and Albert DePina (uncredited)

This was the first time Tarzan had appeared without Boy since 1939. 

Tarzan and the Mermaids was Weissmuller's last appearance as Tarzan

It was filmed in Mexico. Interior and underwater tank shots for Mermaids were filmed at the Churubusco Studios in Mexico City. Exterior scenes were shot at several locations in and around Acapulco on Mexico's Pacific coast.

The exteriors of Balu’s temple were shot at the Aztec ruins of San Juan de Teotihuacán.

Tthe film was in black and white and did not take advantage of the full beauty of the Mexican locations

The film shoot experienced numerous problems in Mexico: Sol Lesser suffered a heart attack  and had to return to Los Angeles ~ inexperienced Mexican crew ~ bad weather, including a hurricane that destroyed the sets

The battle with the giant octopus was recycled in Tarzan and the She-Devil (1953)

At 44, the ex-Olympian, one of Hollywood's most active 'party animals', was long past the slim athleticism of his youth, and looked tired 

Johnny Sheffield, 'Boy', had grown to manhood, so he was written out of the script, under the pretext of being 'away at school'

Monogram Studios immediately signed Johnny Sheffield to star in their series of Bomba, The Jungle Boy films

Sol Lessor's million dollar budget could not overcome an idiotic script, shooting beautiful Acapulco in black and white, a hurricane that destroyed some sets, poor acting and the many other failings of this production.

Johnny Weissmuller fell in love with Acapulco, Mexico and he later became a part-owner - with John Wayne - of the Los Flamingos hotel in “Old Acapulco.”

The filming of Mermaids more of less coincided with Acapulco’s Hollywood heyday when such stars as John Wayne, Gary Cooper, Richard Widmark, Errol Flynn, Red Skelton, Elizabeth Taylor and Lana Turner visited the resort every winter, adding panache and glamour to the scene

Johnny Weissmuller's stunt double Angel Garcia was reportedly killed while diving from the cliffs at Acapulco.

Weissmuller went on to make several Jungle Jim movies for Columbia, His film career ended in 1956 following a brief stint in television. A number of entrepreneurial projects didn’t pan out; his money evaporated. He had married five times. His third wife, Lupe Velez, the Mexican Spitfire, the most tempestuous of his unions, divorced him in 1938. She committed suicide six years later at 34.

In 1973, in the final ravages of fame that also befell actor George Raft and boxing great Joe Lewis, Weissmuller worked as a greeter at Caesar’’s Palace in Las Vegas. A broken hip led to a series of illnesses and he went back to Acapulco to retire at his round-house in the Hotel Los Flamingos. He died in his sleep on January 20,1984, five months before his 80th birthday. 

Weissmuller died of pulmonary edema (water in the lungs) at his home in Acapulco on January 20, 1984.

Johnny is buried in the Jardines del Tiempo (Gardens of Time) cemetery just outside of Acapulco

Co-star Linda Christian was the only person from Johnny's Hollywood days to attend his 1984 funeral in Acapulco

Former actor John Gavin attended the funeral in his official capacity as U.S. Ambassador to Mexico

In accordance with his wishes, a recording of Weissmuller's famous Tarzan cry was played as his coffin was lowered into the ground.

Johnny's name is incorrectly spelled on the plaque at the cemetery: "Weismuller"

The first Tarzan movie, Tarzan of the Apes (1917) starring Elmo Lincoln was shot at the Atchafalaya River delta near Morgan City, Louisiana. A tourist attraction at 725 Myrtle Street now commemorates the spot. 

After Mermaids, Johnny wanted more money to carry on with the series but producer Sol Lesser felt he was too out of shape and hired Lex Barker as a replacement 

Sam Katzman of Columbia Studios hired Johnny to star in a film based on Alex Raymond's comic strip character, Jungle Jim. It was a hit and Johnny starred in 20 Jungle Jim films between 1948 and 1956. This led to a two-year syndicated television series.

 This was the first Tarzan film to make extensive use of singing and dancing. It features a musical score by the brilliant film composer, Dimitri Tiomkin.

 

 
Variety
"Tarzan and the Mermaids" is standard Johnny Weissmuller, differing only from other jungle epics in that this one was produced in Mexico. Also, it introduces Linda Christian to U.S. audiences and is faster-moving than others in the Tarzan group. Robert Florey gets credit for keeping an implausible story moving swiftly with a minimum of dull, hokey interruptions. Sol Lesser moved his company into the Churubusco Studios just outside of Mexico City to film the picture. They also went on location at Acapulco, Mexico's west coast watering resort, for many exteriors. Miss Christian hints possibilities. She's comely and has the physical attributes to measure up for the screen. Thin story is strictly one of those things about a forbidden island in mythical Aquatania where a white trader and his undercover cutthroat employ a fake tribal god to keep the natives subjugated in order to grab pearls. The crooks want Miss Christian as a bride for this god, but she has other ideas. Here's where Tarzan comes in, and where the film gets its tag. He fishes her out of the river accidentally and tells his wife he has bagged a mermaid. Tarzan ultimately unfrocks the phoney tribal priest and his helper but not until after the familiar exciting climax. Weissmuller gets more chances than customary to show his prowess in the water. Brenda Joyce is his very attractive spouse. John Laurenz, as jungle mail carrier, is a pleasing addition with his guitar-strumming and warbling. George Zucco, as high priest, heads the support. Two photographers, Gabriel Figueroa and Raul Martinez Solares, helped Jack Draper on the lensing. Result is some spectacular camera work, probably the best on any Tarzan film.

Variety II
"Tarzan and the Mermaids" has everything that has made the Tarzans a hardy perennial these many years and a bit more. Settings and scenery are spectacular and the thrills include, in addition to the customary jungle action, skin diving and swimming sequences, an underwater battle between Johnny Weissmuller and an octopus and a generous display of slightly-garbed femme shapeliness...  Results, especially the underwater shots are alone worth the price of admission. Tarzans are invariably good box office and this one promises to outgross its predecessors.

The Toronto Star
Tarzan outdoes himself in his newest cinematic exploit, "Tarzan and the Mermaids," with Johnny Weissmuller again in the role of Edgar Rice Burroughs' ape-man and with Brenda Joyce once more portraying Jane. This time Tarzan's adventures bring him into contact with a tribe of amphibious coastal natives, a plot device which gives full scope to Weissmuller's championship swimming abilities. He does more swimming in this than in previous Tarzan films. His underwater rescue of the mermaid heroine and his submerged struggles with an octopus and with a gang of desperate armed natives are thrill episodes. The numerous aquatic scenes make the film very different from its predecessors, but the chimpanzee. Cheta's hamming, and Jane's loyalty remain constant. Linda Christian, features as chief "mermaid" seeking to escape from the brutal white trader who poses as the natives' god, and John Laurenz as a comic singing jungle postman, are excellent in their roles. Edward Ashley, Fernando Wagner, George Zucco and Gustavo Rojo stand out  among the supporting players. Robert Florey has directed the picture with full appreciation of its dramatic value. Much of the Sol Lesser production, including the colorful water scenes, were filmed around Acapulco on Mexico's west coast, with the result that its scenes are spectacular. Note. The above review was lifted directly from the pressbook, which explains its glowing tribute.