You (the reader) want to test it on the local beach to see if it works on multiple targets at once. At this time, only womans were at the beach. For you, it was perfect for the test. Therefore, you commanded the pendant to turn the beach into a mermaid nudist colony and to turn every women on the beach to mermaids.
You see the transformation happening. It happend like this: First, all swimsuits and bikinis just disappear into thin air. Then, they all fell down on the sand as their legs began to transform into fishtails. After that, they all got gills when they saw scales on their legs. Finally, thier legs finished fuseing together to form some beautiful fishtails. They are now mermaids.