Amphitrite: Nereid, or Oceanid. She is represented as the wife of Poseidon and the goddess of the sea (the Mediterranean), and she is therefore a kind of female Poseidon-----by the0phrastus on deviantART
Mavka ( Ukrainian mythology ) is a type of mermaid with long flaxen hair. The name Mavka derives from Nav’ (Navka), which means "the embodiment of death." Mavky (plural) do not have a full body, have no reflection in water, do not cast shadows, and have no back, and so their insides can be seen. ---The Sea Witch Watches with Mirror Eyes Sea
Mythological creatures are cooler when they're anatomically correct
The Fomorians from Irish mythology are steeped in mystery. It is unknown where they came from, or the length of time they had knowledge of Ireland, and what actually happened to them. Lady Gregory describes one Fomorian habitat as a glass tower in the sea. Many people describe them as sea fairies or sea monsters. My best guess is that they were Nephilim!