The seller had to know the boat was junk.
I have to say that I think you are off base in making this assumption. There are a lot of neglected boats out there, that the seller hasn't even looked at in many years. In addition, I can tell you I never inspected any of the boats that I've owned, during the time that I owned them, as carefully as I did before I bought them. What's more, you inspected the boat yourself, probably more closely than the seller has in a long time, and YOU couldn't tell that it was junk. So how can you be so sure that he knew?
Beyond that, I think I'm beginning to get the picture. You could tell that there was a serious problem with the boat. You wanted the seller to tell you just how bad it was. The broker suggested that you should make an offer and get a surveyor to answer that question for you. And you went along.
Sorry to tell you, but you made a mistake. You should not have gone ahead with the survey without getting an answer to your question first. It is most definitely NOT the "normal" process in boat-buying to make an offer when you still have questions for the seller. Perhaps the broker tried to convince you that this is the way it normally works, but it is not.
It is not, offer first, then disclosure later. It is, make a careful inspection, ask your questions, get your answers, make an offer, then get a survey and do a sea trial to reveal whatever might still be hidden. That's the "normal" process.
And yet once again, yes, some sellers and brokers are LMDs. And yet another time, once again, that is a problem with dishonest people, it is not a problem with the boat-buying process. The boat buying process is not backwards.
Edited to add: You also seem to think that surveys are only supposed to turn up "minor," "nit picking" things. Sometimes that is true. Especially if you've done a really good, thorough inspection yourself, and asked a lot of questions ahead of time. Sometimes, though, surveys turn up extremely major things that neither the seller nor the buyer knew about beforehand. This is why you get a survey, after all. If it was a certainty that any unknown issues would be minor nit-picks then it would be silly to spend the money on a survey. The whole point of a survey is to find out if there are any major problems that are hidden.
Like jorgenl, I honestly mean no offense, but you really do seem to have some confusion about how the boat buying process is supposed to work.