Amphitrite was one of Poseidon, the sea god’s, 50 sea nymphs/spirits. Poseidon could have chosen any other sea nymph to marry, but he only wanted to marry her. Amphrite was gorgeous, after all*. One day Poseidon saw Amphitrite and thought he could just go over there and carry her off to be married. But she fled because she knew that, like all the other gods, he would cheat on her and marry other women. Poseidon looked and looked and looked, but he could not find her.
He went back to his underwater palace and ordered a dolphin to go find Amphitrite and bring her back. The dolphin did what it was told because you should never disobey a god. The dolphin found Amphitrite and brought her back to the palace, where Poseidon and Amphitrite were married. Amphitrite became the minor goddess of the sea. The dolphin was rewarded by Poseidon and was made a constellation called the Delphinus constellation. In those times, Greek gods would sometimes make their favorite heroes into constellations as a memorial for what they did. Later in mythology history it is said that Poseidon was in love with a mortal named Scylla. Amphitrite got jealous, and she put some magical herbs into Scylla’s bathing pool, which turned her into an ugly sea creature with legs and heads all over her ugly body.
*Sorry if writing about marriage and love is disturbing to you, but a lot of Greek myths are like this (don’t get me started on the myth of Psyche and Eros).