@RMC1 being a demo ski, I have a suspicion on what has happened and willing to put money on it.
Whoever took that ski out for test riding, hit something soft&smooth but with a high density. The dealer would have not noticed because they would have been no scratch. The rider itself might still think that he/she caused no damage because upon visual inspection no damage would have been evident. The dealer sold it to you as a new unit because they rightfully thought there is nothing wrong with it ( unless off course the rider told them of the impact and they went to extensive measures to check the hull integrity).
you took the ski thinking that everything was good as this was a new ski. As you were using the ski during the summer, your usage ( not necessarily rough usage) during the summer made the damage visible. I am very familiar with this chain of event both from my boat and friends boat. During our 2017 crossing to Bimini our boat had damage, but some of it was not visible. We come back to Florida and no visible damage was seen. When we boated back to Texas ( and PA for my friend) we noticed the damage the next day. The vibrations from the towing had exhasstrubated the damage.
My boat still bears the 2017 ( as the damage is above the water line and I was not too worried about making a perfect repair) and my friends boat had to go to the show for months. How her boat looks awesome but that is different story.
Although, a speculation, I am fairly positive that is what happened to your ski. The "diffusion" pattern on your damage is what you would see from high speed impact.
As per your existing situation, I think you are in bit of pickle situation. You cannot proof that you did not cause the damage and I will be shocked if Yamaha will take this as manufacture defect. The dealer is not likely to take the ski back or admit to any wrong doing, because at the end of the day it is your word against all the people who try to work the system. If you do not want to pay our of pocket, you best bet is the insurance. The fact a bunch of people on public forum are saying that the Yamaha hull suck, cannot stand in court. That is just BS with them trying to get off the claim.
I personally use insurance only in catastrophic events( ie all my vehicles carry only liability including a 2019 CRV) because every time you file a claim the insurance alogarythm adjust your risk of future claim. Which in turn makes your premium go higher for your vessel and likely for all other things you have insured. There are different opinions how insurance should be used, but in limited knowledge of risk that is how things tent to work. At the very least, now the season is over you can take it to shop to give you an estimate. Then you can decide to go through insurance or take a loss of whatever.
In regard to your wife's claim, I believe it is probably exaggerated, but justified. I will explain why. In my book, that ski can be repaired. Not necessary by replacing the hull, but with an actual repair. The work needed to replace the hull it will not be worth it. Statistically speaking if a professional does the repair, it should be be good for the 99% of the people ( including your family I would say). If you take the kids to a lake and tube etc, the change of your family loosing its life because of that damage is like being strike by lightening. At the end the day if the ski sinks, y'all should have lifejackets one and will be picked up by the next boat that will drive by. Just a bad day at the lake.
So why I think your wife is justified. Well because she needs to be comfortable. If she is the overprotective type and she wants fun, she will not one able to relax if she keeps thinking that there was damage on the ski's hull. You might never have an issue with the repair (which I believe will be the case), but she needs to be able to be care free.
As per your last comment on taking a loss on it, I think your are just frustrated. These thinks are too fun much to pass on them because fate played a cruel joke on you. That ski can be repaired and there is no reason get rid of it. If you want a peace of mind and not worry about it, put for sale here as is and make sure you ping me because I bet it will sale quickly