I've been thinking about how to do reefs in general myself.

I've seen hand drawn images of old showing them but it seemed the reefs where the point of the map so that's really not what you want here (unless I miss your point).

If I were experimenting on that I would try a few mockups:
1. Draw them using a finer point
2. draw them using a blunt/fuzzier point
3. draw them at a lower opacity
4. shading the entire shoreline leaving reef areas white thus indicating them by omission.

Mind you, I haven't actually tried any of these yet but that's what I would try if I were. It's quite possible none of them will work.