The best replacement for mermaid is siren.

The best alternatives to mermaid are rated as following:
★★★★★  siren
★★★★  whale
★★★  trumpet
★★★  sailor
★  reverse mermaid
★  fit and flare
★  seal
★  merman
★  trumpet dress
★  ball gown
★  trumpet style wedding gowns
★  trumpet wedding dress
★  manatee
★  sailor marina and the diamonds
★  trumpet wedding dresses
  mermaid
  fit and flare wedding dresses
  beauty and the beast
  drop waist
  trumpet style wedding dresses
  lion king

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Speaking about mermaid, one probably shouldn't forget about anchor tattoo.

Other search terms related to mermaid and their ratings are:
★★★★★ anchor tattoo
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Google Trends

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mermaid
siren
whale