You shall sail the iron ship with warriors made of bone,
You shall find what yo seek and make it your own,
But despair for your life entombed within stone,
And fail without friends, to fly home alone."

​The first line of this prophecy refers to sailing the CSS Birmingham (a Confederate Ironclad raised by Ares) with a crew of undead Confederate soldiers who owed a tribute to Ares for being on the losing side of a war.

The second line refers to Clarisse finding The Golden Fleece and taking it from Polyphemus

The third line is a warning against being trapped in Polyphemus' Cave in the same way Polyphemus trapped Odysseus in by sealing them in with a boulder.

The fourth and final line is quite literal.  It means Clarisse would fail without friends (Percy, Annabeth, Grover Underwood and Tyson) and that she would fly back on an airplane to New York alone.