In Greek mythology, the Nereids are sea nymphs (female spirits of sea waters), the 50 daughters of Nereus. The most notable of them is Thetis, wife of Peleus and mother of Achilles.
They symbolized everything that is beautiful and kind about sea. Their melodious voices sang as they danced around their father. (Wikipedia)
Nerissa is a name created by Shakespeare for a character in his play ‘The Merchant of Venice’ (1596). He possibly took it from Greek Νηρεις (Nereis) meaning “nymph, sea sprite”, ultimately derived from the name of the Greek sea god NEREU. (Behind the name.com).
Other mythical sea creatures include the Celtic selkies, French melusines*, ondines, and little mermaids.
*I studied Melusine by Jean d’Arras at the Sorbonne in Paris (shout-out to Sara Kleinman for suffering through that lecture with me)