We tend to underestimate the Sea Gods. In Dragon Pass, the main water cults are Heler the Rain God (practically a subcult of Orlanth Thunderous), Engizi/Sky River Titan (because of the Skyfall is closely linked to Heler) and Oslira (in the Grazelands and Tarsh). As important as these deities are for agriculture and such, they are overwhelmed by Orlanth, Ernalda, Yelmalio, and even Kyger Litor (who is manifest in Dagori Inkarth).
When we get to the Holy Country we get Dormal, who is the God of Sailors, and pleasant Choralinthor, who blesses the waters of the Mirrorsea Bay. Useful deities, but they hold few cosmic secrets.
But we often forget that more than half the world is covered by waters. And those waters go deep – the Homeward Ocean has at its center the great whirlpool formed by Magasta that goes to the Underworld and deeper.
Within the Deeps are deities of great power that are rarely worshiped (or even known) by humans. Greatest is Magasta, the Lord of the Waters. There is Wachaza, his son, a terrible and cruel god of Death. There is Triolina, the Source of Life. There is Daliath, Guardian of Secret Wisdom, and Nelat, the God of Purification. There are great spirits that animate the waters – ruled by a deity popularly called King Undine. There are the gods of the various seas, who are the master of the currents and the waves.
These deities are mainly worshiped by the Triolini, and possess secrets unknown to the humans of the Third Age. There are the Waertagi, who have returned to the seas, seeking vengeance upon the land humans that banished them with the Closing.
And within the Deep is Chaos as well. The endless hunger of Gloomshark, the End of All Things. The great white shark with dead eyes that consumes all. With the triumph of Chaos, even death may die.
For humans, the oceans are a place of deep secrets and terrible powers, a vast world largely hostile to humanity.
In Dragon Pass, Orlanth Thunderous and Ernalda bring forth life. Orlanth Thunderous brings forth the rains (who as rain is called Heler, Orlanth’s loyal servant). Irrigation is largely from the local brooks and creeks, not from Engizi, and so we just try to make good with the local nymphs and offer them gifts each year.