If you're talking about the geographical feature:
* Deep water: This is the opposite of the shallow, dangerous water found around reefs.
* Open sea: This refers to the vast, unhindered expanse of ocean far from any reef structures.
* Plain seabed: This refers to a flat, unremarkable seafloor lacking the distinctive features of a reef.
If you're talking about the reef as a barrier:
* Passage: This suggests an open, navigable route, the opposite of a reef's obstruction.
* Clear route: This emphasizes the absence of any obstacles or barriers, in contrast to a reef.
* Free flow: This implies unimpeded movement, the opposite of the restriction a reef can cause.
If you're talking about the reef as a structure:
* Plain: This signifies a lack of complexity and intricate formation, the opposite of a reef's intricate ecosystem.
* Unstructured: This refers to an absence of the defined, organized shape of a reef.
* Unpopulated: This emphasizes the lack of the diverse life found in a thriving reef.
It's important to consider the specific context when choosing the best antonym for "reef."