"but that does not mean that he would not have enjoyed the other boats equally or better and since he has no real point of reference, would not have found the other boats much better suited to his long term goals."
It doesn't mean the opposite either. Only that person, in retrospect can answer that question. And to actually do that, one must step off the dock onto some sort of boat, your advice not withstanding.
"I am clueless as to why he chose that boat over the better maintained and updated choices that were out there at the time."
Exactly. There is no way you could possibly know what's right for him, since, unless you've figured out the Vulcan mind-meld, you cannot truly know his thoughts and motivations.
People are often impressed with pages of information, regardless of their level of comprehension. It doesn't mean the question has been answered. A person could come into a perfectly good boat, only to have you convince them it's useless or outright dangerous because it follows a for you don't like.
That's where it gets sticky, because you let your opinion of what's best for someone else get in the way. That's what incites others to come in and say, "Now wait a minute......."
Instead of just redirecting people, or flat turning them off, why not offer advice on how to ease the difficulties you've experienced on a particular boat since some will disregard your wave-offs anyway?