How should a reef be tagged. natural=reef seems obvious, but I do not think it is defined. I had a look at the Great barrier Reef, but that is not mapped yet. No doubt there are other marine features that could do with their own tags. natural=sandbank for example (Goodwin Sands etc).
If natural=reef seems like natural tagging for that feature, you can just start to use it.
For best chances of adoption by other people, you can make a proposal on the wiki, or be otherwise vocal about it.
I did manage to find a proposal for sub_sea=reef, but that does not appear to have made any progress. natural=reef is also mentioned in that. I will look at making a proposal for natural=reef.
Draft proposal started here http://wiki.openstreetmap.org/wiki/Proposed_features/reef
natural=reef sounds like a very sensible tag so I doubt you’ll have too much opposition to it. Regardless you can and should start using the tag right away.