* Quay is a specific noun referring to a concrete or stone structure alongside a body of water for loading and unloading ships.
* The opposite of a quay wouldn't be another structure, but rather the absence of one.
Instead of a direct antonym, you could use words that imply the opposite of what a quay provides:
* Shore: A natural, unconstructed edge of a body of water.
* Deep water: A location far from the shore where ships can anchor without needing a quay.
* Open sea: The vast expanse of ocean, where ships are not moored or docked.
The best word to use depends on the context of your writing.