Problem with artifacts using Approximate AO

I use Approximate AO quite a lot in Blender, but for the first time I’ve noticed odd artifacts that it seems to be producing (more noticeably without OSA):

If I disable shadows then the artifacts appear like this:

With shadows on, the problem seems to be triggered by the sea plane’s material having its ambient slider at 0.0. Raising it to 0.1 makes the artifacts go away. I’ve tried this with several other objects in the place of the little boat and they also showed odd artifacts.I can’t work out what the matter is with shadows disabled.

I seem to be able to work around this problem, but I’d like to know the cause. Is it a known issue?