I feel bad for you, the OP. However, this reminds me of daytime court shows. Girl buys car from guy she trusts who says it's a good car. She drives it and starts having trouble, takes it to a mechanic, who points out all sorts of things. She loses because the didn't use a mechanic before buying and protect herself.
You likely lose for the same reasons. Was there any warranty given to you? If not, then you have no recourse unless there was fraud. I know you pointed out the water issue was known by the salesman, but doubt he knew the cause and if you can't prove he did, there's no fraud there. You accepted the boat delivery in that condition.
You took shortcuts. No survey. Not even a personal inspection. While the issue and the video and the subsequent survey may reflect poorly on the builder and on the dealer, that really doesn't help you.
Now, you need to find someone to tell you what needs to be done and repair the boat. If they know what an issue you're making of this and how you're badmouthing the dealer and the builder, no way most are going to want to do work for you. You best get off your high horse and go to one on your knees begging for help.
Three wrongs don't make a right. Perhaps the builder did something wrong, perhaps the dealer did, but definitely you took short cuts. There is no way to know what happened. Something might have happened with the first owner. Perhaps even before they bought it. Just takes one yard incident. As to the thickness of the fiberglass and things like that, you weren't buying a Hatteras. Ranger is built light and that includes thin. Nothing inherently wrong with that. The boat we're on right now is built lighter than any others we own. Hopefully materials that will sustain but built for speed. I quote from their website, "Advanced construction technologies associated with the use of lightweight materials everywhere."
Reality is you're fighting a losing battle and continuing to wage the fight will only cost you, in time, energy, money and reputation. You can't win. You bought a used boat and have problems. You cannot possibly know when the damage occurred. The boat traveled a great distance on a trailer after you bought it. Maybe a speed bump. Who knows. Perhaps the previous owner had it hauled and blocked. Perhaps when new at the dealer. Is it faulty construction? Doesn't really matter and by what standards? You have no recourse and likely it's their normal standards.
I can understand your pain and your anger. I would be furious at some of the treatment. However, it's time for you to direct all your energies to a solution. If you had insurance at the time of transit, I frankly would have filed and let them determine whether to deny as a manufacturing defect. I wouldn't have said anything untrue, just that I bought it and it arrived and has this damage. At least they would have paid to have it analyzed. You though only have three choices. You can sue, but you have no case that I see. Second choice is to sell as is. Third choice is to get it fixed and then decide further. Deep down, you know you screwed up. Forgive yourself and move on. We make mistakes and they're painful. Sometimes they even involve being taken advantage of. Still we learn and move on.