Dang that sucks. Had almost the same thing happen to me about 3 or 4 years ago with my Supra, your damage sounds about excatly the same. I was able to hobble back to the launch but couldn't barely even turn the steering wheel, and my driveshaft was bent. Still never figured out what it was, but assumed floating log or something (was in a river). Oh well, same thing for me, it was the end of September and insurance covered it so no real harm to me other than $500 deductible, but needed a new prop anyway!

I can't believe it's that shallow and nobody has ever marked it.