Surely the mark is a power driven vessel and is under command albeit remotely

Beau .. couple Q's/observations ..

First, it's always been a question in my mind to the extent that NOR/SI can refine/define a term not in Definitions. In you instance, you aren't actually doing that directly (but may be indirectly) when one states a certain area is "not navigable" when it has "plenty of water" for a boat to passage through. We had a thread a while back exploring that .. I'll see if I can find it. 

Q: Do any of these obstruction-zones have a starting mark at its perimeter?  That would put your example closer to the OP. 

Think this one deserves its own thead to get folks opinions. Standby. 

You have made me realize that I spend too much time practicing starts, and not enough time practicing finishes.

Yes John, I agree. I meant there may be no reasonable opportunity on some courses and so there seemed to be some suggestion that the protest needed to be raised before the NSC boat crossed the finishing line and I was merely trying to say that couldn't always be possible.