Fangy and I went looking for penny's and grumpynuts's rocks, couldn't find Penny's (might have been the same disappearing rock I hit).
But we're glad we found Chris's straight away. Admittedly it was very low tide, but it was about a centimetre out of the water, in water about knee to thigh deep.

The post is about 2 metres west of the rock, that's as close as I could get the post in the ground, so don't go too close, and hopefully it won't claim any more boards!