Here's Genie Bouchard proving two legs are a helluva lot better than one tail ... 'cause she ditched her mermaid outfit for a tiny black bikini Wednesday -- and we're all grateful for it.
The tennis superstar has been over in Necker Island for the week ... splashin' around in paradise -- mostly in barely there bathing suits.
But, today ... Genie broke out a mermaid's tail -- and while she looked smokin' hot in it -- when she bailed on the fit ... the pool she was in definitely got a little bit hotter.