This is one of my favourite of Zelazny's short stories. It's set in a futuristic, semi-terraformed Venus with mighty oceans, where extreme sportsmen chase after gargantuan fish. (Gargantuan as in, kilometers long.)
From this story stems my abiding love for huge monsters in large bodies of water. (Although I believe there is also some deep psychic human terror of the sea, the ocean, and big lakes, which makes the Sea Monster trope especially interesting and frightening. There's a reason why things slumbering at the bottom of the ocean are scarier than those slumbering underneath a mountain.)
The best huge-monster-in-large-body-of-water is of course the kraken.
Kraken in Yoon-suin are demigods, worshiped by the squid men of the Great Blue Reef. Many are hundreds of metres long. They drift through the ocean, feeding on anything they can find - insatiable, horribly intelligent, cruel. The innumerable prayers and sacrifices sent their way by the squid men give them energy and power, and they grow fat with divine puissance. They grant fear- and pain-causing spells to their followers, and delight in the destruction these powers cause. And sometimes, when the call is strong enough, they can be summoned to the coast to feast on their worshipers' enemies.