Those cops are going to have one hell of a mystery on their hands:
"wait she drowned there? it was just days ago she mentioned having seen a mermaid at that spot!"
"my daughter looked up some suicide of another girl at that spot the day before!"
forensics: "uuh, the hair wasn't all we found. She also had he cheek ripped off her face by human teeth!"
"Hey, there was a notice to call in if we had witnessed anything about that case, yeah? I saw that girl standing by the shore seemingly captivated by something in the waters, and pulled her back to land and warned her about last years suicide (didn't figure out why someone else intentionally taking their life there made the spot dangerous, but still!) just 2 days before"
More like Siren, though some types of mermaids had old-school lore like you mention (didnt jp culture specifically do so?), and iirc early christianity mermaids were depicted as such too (like how they did for pretty much any and all pagan folklore that wasn't needed to aid converting others (such as some saints being traced to pagan gods or spirits) and better vilified instead (due to not fitting nicely, or better if stamped out properly, or simply because the missionary in question had their own opinions).