The next question is 'Is a [starting] mark that is not surrounded by navigable water, and is thus  an obstruction,  necessarily a continuing obstruction?'

Let's consider the OP situation.  The sunken  object is 5 ft in diameter.  The distance between the outside of the channel marker (assumign also a 5 ft diameter) and the outside of the sunken object is thus 25 ft.

We can now apply the definitino of continuing obstruction 'at least 3 hull lengths'.  In the OP case we have a boat of hull length 35  ft, so the channel marker and the sunken object combined are less than 3 hull lengths and therefore do not constitute a continuing obstruction.

So, considereing the applicability of RRS 18 at a start, and assuming that we agree that the mark is NOT surrounded by navigable water:

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.