Racing on San Francisco Bay, we have numerous "exclusion zones" which create obstructions and continuing obstructions between things, these are defined by a line or set of lines that exclude boats from sailing through the area. They are obstructions by being included in a rule. Heaps of people call for "room" to tack at these created obstructions and continuing obstructions. (see: Definitions Obstructions and Continuing Obstruction) That said, we are careful to have the boundaries of these exclusion zones be far outside of the actual obstructions or continuing obstructions.

I don't think you wish to: "...include the float in the definition of pin/mark, thus make the pin/mark surrounded by navigable water." What constitutes "navigable water" depends on the boat and a number of other factors including sailing skill, sea state, hight of tide, etc... 

Rather, I'd suggest the NoR and the SIs reference the described white mark with a description of what it is alleged to be marking. I would probably include a statement along the lines of: "CAUTION: A white ball has been reported in the area of XXX longitude and YYY lattitude. It is reported to mark a sunken marine hazard. Competitors are alerted that it is near the Pin End of the starting line." I would also verbally mention this at the Competitor's Briefing.

I would not go further in an attempt to engineer the competitor's behavior, and thereby take some responsibility for a boat's safe navigation near a marine hazard.

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.