My PM did not get out of my Outbox yet, so here it is in full:
Since the board has already measured your boat, any procedure such as removing the rubrails will be viewed with suspicion and almost certainly called into question.
I suspect many boats advertised as a beam of 8' are not exactly 96.00 inches wide. I would measure all boats that already have slips-or just ask the board if it has done so--to determine if they all are 96.00 inches or less by the same unforgiving measurement standard. If any is not, is its slip license being revoked? Any grace period to get a conforming replacement boat? If the existing boats are permitted to keep their slips, then they are "grandfathered" in under the by-law (but the by-law should be explicit on this point). If you have been on the waiting list with the same boat, then you could argue it is unreasonable discrimination to grandfather in the slipped boats and not yours, particularly if the by-law language on grandfathering is not specific on this point.
You could also check to determine whether the bylaw was properly adopted, with notice, adequate description of the action, etc. You presumably got notice of the bylaw proposal but did not object because you owned an "8 foot" wide boat. Even if not a good legal argument, there may be sufficient other boaters in your situation that would side with you (as well as those who would not mind seeing you disqualified once they check their measurement). Here, you can argue fundamental fairness to the board and ask for a clarification based on your manufacturer specification and being misled by the bylaw proposal, assuming it was not specific on the measurement standard. Also, depending on the layout of the slips (one boat per slip, perpendicular; two boats side by side) the rubrail width may not make a practical difference to achieving the board's objective. Fundamental fairness and a reasonable position is your best chance.
Finally, I believe there is an ABYC standard on boat dimensions that fixes three different width determinations: basically, waterline; structural hull (including deck to hull joinder); and with everything else, such as non-structural rubrails, added on. So the board's measurement standard does fall within one of the three approved methods (everything else). You can find the complete standard here:
https://www.abycinc.org/committees/s-08.pdf
None of this is legal advice, but just a practical view on your alternatives. If you can make the discrimination argument and the board does not accept it, then you might discuss it with a local lawyer (preferably one in the association with an 8'2" wide boat). Sometimes just having a lawyer speaking for you can make a difference, without having to resort to expensive litigation that also would have ripple effects beyond boat width and slip rights.
Good luck.