Thank you Phil and Ant and John.

Yes, my logic statement is murky in part because it is naturally read as "if and only if," but I did not mean that. 

Let me try to narrow down the issue I'm having. 

Let us set aside the question of obstruction/continuing obstruction. I think that question is (almost) a red herring because the pin/mark (or pin/mark/float combination) is almost certainly not a continuing obstruction. 

Let's just assume the critical question here for preamble purposes is whether the pin/mark is surrounded by navigable water.  The answer to that is: "depends".  Got it.

But it would be much better if the racers had certainty about whether Rule 18 applies at this pin/mark.  And, I think you both agree (not sure) that it is usually better for Rule 18 to NOT apply.  The SI's can be used to create certainty.  Oddly stated, perhaps, but the goal here is to use the text of the SIs to surround the pin/mark with navigable water.

I have a problem with Ant's proposed solution, which is to say in the SIs that boats may not cross an imaginary line between the pin/mark and the float.  In my mind, this does not solve the problem because the pin is still not surrounded by navigable water.  My proposed SI edit would include the float in the definition of pin/mark and thus make the pin/mark surrounded by navigable water.  

The pin is the pin.  The float is the float.  The mind sort of naturally conflates them, but they need to be addressed separately.

Pretty wild stuff, but am I wrong?

Welcome Charlotte!  Happy to have your contribution. 

Ang

As a retired IJ and IU my take is that Blue, being a fast reaching multi-hull, is on the cusp of breaking rule 17 at position 6.

I think that wording is quite workable, if that's what you want to do.

Rule 89.2 provides the option to change the NoR provided adequate notice is given.

As brought out by John d Farris, any change needs to be communicated to all stakeholders.