Greek sea spirits* of old.
In Greek Mythology, the Nereids were the fifty daughters of Nereus and Doris. They lived in the Mediterranean Sea, particularly around the Aegean Sea, and gave aid to the Greek sailors. The had the ability to shape-shift at will.
They pop up all over in Greek Myths, but usually only as bit players. Amphitrite is probably the best known. Thetis is often said to be their leader.
Some of the others Nereids were:
Agaue
Aktaia
Amatheia
Amphinome
Amphithoe
Apseudes
Dexamene
Doris
Doto
Dynamene
Galateia
Glauke
Halia
Iaira
Ianassa
Ianeira
Kallianassa
Kallianeira
Klymene
Kymodoke
Kymothoe
Limnoreia
Maira
Melite
Nemertes
Nesaie
Orithyia
Panope
Pherousa
Proto
Speio
Thaleia
Thoe
(Iliad 18.37-50)
To Nereus and to Doris... there were born in the barren sea daughters greatly beautiful even among goddesses: Ploto and Eukrante and Amphitrite and Sao, Eudora and Thetis, and Galene and Glauke, Kymothoe and Speio, and Thoe and lovely Halia, Pasithea and Erato, Eunike of the rose arms, and graceful Melite and Eulimene and Agaue, Doto and Proto, Dynamene and Pherousa, Nesaie and Aktaie and Protomedeia, Doris and Panopeia, and Galateia the beautiful, Hippothoe the lovely and Hipponoe of the rose arms, Kymodoke who, with Kymatolege and Amphitrite, light of foot, on the misty face of the open water easily stills the waves and hushes the winds in their blowing. Kymo and Eione, Halimede of the bright garland, Glaukonome, the lover of laughter, and Pontoporeia, Leagore and Euagore and Laomedeia, Poulynoe and Autonoe and Lysianassa, Euarne of the lovely figure and face of perfection, Psamathe of the graceful form and shining Menippe, Neso and Eupompe, and Themisto and Pronoe, and Nemertes, whose mind is like that of her immortal father. These were the daughters born to irreproachable Nereus, fifty in all, and the actions they know are beyond reproach, also."
--Theogony 240-264
* They are probably technically sea goddesses, rather than sea spirits, but the terminology is in this area is muddled.
For some good lists of and information about the Nereids, check out:
http://hsa.brown.edu/~maicar/NEREIDS.html
http://www.theoi.com/Pontos/Nereides.html
http://www.1freespace.com/women/alexiares/nereis.html is a great site that lists 80 (yes, there are only supposed to be 50, but they aren't always the same 50 in every story) Nereids, and translations of their names.