Born old and having to share a single eye and tooth, Deino, Pemphredo, and Enyo were triplets who guarded either the location of or how to defeat their other sisters the three Gorgons. The hero Perseus, on a quest to kill Medusa, forced the answer out of the Graeae by stealing their eye and tooth.
The Graeae
Scylla was born as a beautiful nymph loved by the sea-god Glaucus, but turned into a monster by a jealous Circe. Scylla and her whirlpool counterpart Charybdis resided in a narrow channel of water, so narrow that all passing ships had to go near one of the two monsters. Scylla would devour six crew members, but Charybdis would destroy the entire ship.
Once a beautiful naiad, Charybdis assisted her father Poseidon in his feud against Zeus by flooding huge swathes of land. Charybdis was so successful that Zeus turned her into a monster to stop her. While unable to claim anymore land in this state, Charybdis continued to cause havoc by creating whirlpools - specifically by sucking in and vomiting out the sea water thrice a day.
Two of the three Gorgon sisters, who would turn all that saw them into stone. Unlike the more famous Medusa, Stheno and Euryale were immortal, though were notably pained by her death.
The third Gorgon sister, and the only mortal one. As a Gorgon, anyone looking directly at Medusa would instantly turn to stone, although looking indirectly (such as her reflection) would have no ill effect. The hero Perseus was able to decapitate Medusa and use her head as a weapon -- even in death she could still petrify.
One of the Twelve Olympians and ruler of the seas, Poseidon led an active life and played a part in many myths, not all of them strictly tied to the oceans. Among other things, Poseidon created horses, caused earthquakes, and chased women.
