We are never told--what was running through Poseidon's mind as he left Percy's mother? How could he leave her, knowing she carried his child? I've attempted a speculation...

The Parting

Poseidon felt the coolness of sea water slipping against his skin as he navigated through the violent waters. Waves crashed, churning the water below the surface, and yet the god of seas did not waver. Much was on his mind, easily reflected in the turmoil of the surface of the waves. The sea was unpredictable, often unreadable, as was Poseidon. But today the sea god was tormented by his obligatory return to duty.

He was leaving behind so much, yet the sea beckoned to him, calling him back to his watery abode beneath the surface. And so he continued on, slicing through the dark waters, yet his mind was preoccupied with someone far behind him. The god who could stop the tides on a whim, inflict earthquakes and instigate hundred-foot waves could not return to what he had just lost.

A thousand miles west, a beach encountered a similar storm. The waters were furious and unrestrained as they swept against the sandy expanse with equal fury. Upon the shore, a woman stood ankle-deep in water, the waves raging against her like a promise of plight. The wind whipped her hair, brushing it off her bare shoulders to dance about her, her eyes stormy as the cloudy grey skies. Her sundress seemed wildly out of place, but somehow, it seemed to give her light underneath the darkened skies. Rain leapt from the clouds, streaking down her face like tears. She placed a single hand on her stomach, the tears real.

"Poseidon," she whispered. Across the ocean, a god turned, but could not answer.