I guess it depends how much you know and the consequences if something goes wrong. THe most common example on this forum is when someone comes in asking how to make a 20kV Tesla coil, but they obviously don't know what they're doing so you get all the "safety nazis" coming out. If that is the case, then making them feel discourage *IS* the goal since they're dead if they are lucky and get it even get it partially working.
But when someone who seems to know what they're talking about asks, people just answer their question. THis happens fairly rarely though since the people who know enough to safely go about making 20kV Tesla coils usually don't have to ask anything.
At least for me, the "scrap metal is scrap metal" argument doesn't really apply. Maybe it's just the nature of the material on this forum, but I've never noticed anyone really concerned about about issues like "resale value" or material losses (short of things like burning your house down, destroying your car engine, or sinking your boat). Usually the problem is the person killing themselves or doing something illegal like cluttering the radio waves.
I tend to not to shoot holes in ideas I don't know much about because how can you when you don't know much about it? So I'd assume the people who are shooting down your idea at least know SOMETHING and what they say has some merit. I'll admit that it can be satisfying to shoot holes in bad ideas but remember that it's also really easy to convince yourself that other people are jealous, or too afraid , or not as innovative when they start saying things about your idea that you don't like. It happens all the time on this forum with people who have "top secret projects" involving their own patents.
Then again we are talking about a boat, and not something small like a PCB.