A warning to anyone unfamiliar with Narrabeen Lakes - beware the invisible sandbars!

I went out there for the first time yesterday, marginal and patchy NE wind, underpowered on a 7.5. I'm finally properly powered up and going about 20kts when I hear the dreaded scraping sound from the 40cm fin. No time to unhook - about half a second later the board stops dead and I fly forward, still hooked in. I've never catapulted onto only-just submerged solid ground before and don't recommend it. Somehow I escaped with only a few cuts and bruises on one arm; the boom is written off but no other kit damage.

I now figure the yellow flag I had assumed to be a marker for a race that was going on is actually indicating the end of the sandbar...

The sandbar is obvious on the satellite view:


One bent boom: