Oceanids are the children of Oceanus.
All nereids are nymphs, but not all nymphs are nereids. The difference isn’t just, “water nymph”. Nereids are the children of Oceanids.
(Naiads are basically the same except fresh water. Naiads are the daughters of the Potamoi, who are children of Oceanus :D)
Penelope is a nereid. Amphitrite’s grandfather is Oceanus, so she’s also a nereid. Penelope and Amphitrite are cousins.
Do with that what you will.
Edit: Ask and you shall receive. Screenshots from my reference folder:
Cousins.
In the beginning there were Chaos and Nyx and the black Erebus and the wide Tartarus, but there were not Earth, nor Air, nor Sky; and in the wide hole of Erebus, first of all Nyx of black wings created an egg raised by the wind, from where in the cyrcle returning seasons bloomed Eros the Desirable, with his refulgent back from two golden wings, similar to speedy whirlwinds. And he at night blending with winged Chaos, in the wide Tartarus, hatched our people, and firstly brought to the light. Until then there were not immortal seed, before Eros mixed everything; but being mixed each other, birthed Uranus and Oceanus, and Earth and seed without destruction of all blessed Gods.
Aristophanes, The Birds - 693-702