I thought this was simpler, at first.
The SIs have gotten close to changing (or trying to change) part IIC rules. The SIs designate the line between I and the breakwater as an obstruction that shall not be crossed while racing. Boats are "racing" after their prep.
The preamble to section IIC says that they do not apply when the mark is a starting mark and the boats are approaching to start. By declaring the line an obstruction that cannot be crossed while racing  they are attempting to change section IIC.

I think the prudent thing would be for the SIs to be specific.
For example, if the intent was to make mark I a normal starting mark, change the wording defining the obstruction that shall not be cross "after starting and before finishing." 
If that is the case, 18 and 19 do not apply when the boats are approaching to start. 

If the intent was to keep boats out of that space at all times because of safety, a course chart in the SIs should declare the area non navigable. We sometimes do that for areas where the Navy may be operating. Failing that, I think "navigable" has to be interpreted with the common-use definition. You specify that it is navigable under normal situations. Thus, Section IIC does not apply. 

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. 

I have just watched the Women's regatta on mastabeam. Fantastic.