|Definition:||A rock, sandbar, or other feature lying beneath the surface of the water (sea)|
|Rendered as:||Area:Lighter than sea (coastline). e.g. leisure=playground. Node:Just use name|
A reef is an area of sea bed which can be made from rock or possibly a sandbar. It is usually just beneath the surface and may be exposed at low tide levels.
Wikipedia provides a detailed explanation of a reef 
There is currently no suitable tag for a reef. Reefs are popular destinations for diving/snorkelling. They frequently pose a hazard to shipping/marine activities.
How to Map
Example & Description
The Great Barrier Reef is famous throughout the world as an area of natural beauty. Popular with tourists, divers and marine biologists, it is currently completely unmapped. See  and click on edit to see one area that is unmapped.
Another example here . Again click edit to see what is missing.
Could also add:
- type=fringing - Where the reef is attached to an island.
- type=barrier - Where the reef forms a barrier near an island leaving an area of water between the shore and the reef.
- depth=number - The depth of water above the reef at lowest tide. So if number is 0, the reef is uncovered at low tide.
- artificial=yes|no - If the reef is natural or from a sunken ship for example. artificial=no is assumed if tag omitted.
Rendering could be different for various types and also if depth=0. Note: Parts of a reef that are never covered by water should be considered as land and tagged appropriately.
Alternate Name Suggestions?
- subsea=* - currently ~ 1000 coral reefs mapped that way
- sub_sea=* - currently ~ 600 coral reefs mapped that way
Please use the Talk:Proposed features/reef for discussion.