nereid

noun1

1. •
   (figurative) A youthful and pretty bather.

   • I knew in the back of my mind that the “nereid” wore a bikini intended for photography and that she lived in that bikini all summer. — 2014

noun1

1. •
   Any one of the fifty sea-nymphs who were daughters of the sea-god Nereus and attendants upon Poseidon (Neptune), and were represented as riding on sea horses, sometimes in human form and sometimes with the tail of a fish.

   • No Dryads sought to avert the fate of their kindred trees - no Nereides sported on the waves. — 1828