Fiona, that is exactly what I was looking for. I don't feel comfortable making something up "on the fly" and I feel a lot better having something more formal to say. Anyway, this is what I found, and I like it:
ll. 1-5) I begin to sing about Poseidon, the great god, mover of the earth and fruitless sea, god of the deep who is also lord of Helicon and wide Aegae. A two-fold office the gods allotted you, O Shaker of the Earth, to be a tamer of horses and a saviour of ships!
(ll. 6-7) Hail, Poseidon, Holder of the Earth, dark-haired lord! O blessed one, be kindly in heart and help those who voyage in ships!