Fed up with scientists, hunters, leopard women, Amazon warriors and Nazis ruining their peace and quiet, Tarzan (Johnny Weissmuller) and Jane (Brenda Joyce) have upped sticks and moved to a riverfront property, where they hope to spend their days relaxing, fishing and enjoying a little privacy now that Boy has finally flown the coop (gone to school in England, apparently, although I reckon actor Johnny Sheffield saw the proposed plot for this film and ran a mile!).
This seemingly idyllic locale still has its drawbacks though: not only do the couple now have a really irritating new neighbour in the form of calypso-singing mail-man Benji (John Laurenz), but only a few short vine swings down the river, trouble is brewing in the taboo coastal realm of Aquatania, where a pair of wicked con artists (George Zucco and Fernando Wagner) are posing as living deity Balu to trick the locals out of a fortune in pearls. After giving refuge to a young Aquatican woman who has escaped from a forced marriage to the false god by swimming upriver, Tarzan decides that if he is ever going to get some well earned rest and relaxation (and judging by Weissmuller's haggard appearance, he really could do with some), he must first sort out the unrest in Aquatania (although personally I would begin by throttling Benji!).
The twelfth and final outing for Johnny Weismuller as Edgar Rice Burrough's ape-man, Tarzan and the Mermaids is easily the weakest of the whole series, with a star who looks like he's been partying way too hard between shooting (stubble and a paunch is not a great look), a poor script that favours musical numbers, lighthearted aquatic frolics, scenic panoramas over decent action and adventure, and a thoroughly unconvincing Mexican filming location (an Aztec temple, the unmistakable cliffs of Acupulco, and a supporting cast of Hispanic performers don't exactly give off that African vibe). Director Robert Florey's tepid direction does little to help matters, with even an attack by giant octopus proving to be a big disappointment.
Weissmuller enthusiasts will no doubt want to check out 'Mermaids' for the sake of completion, but be warned, this ain't a great way to end an era.