Mark the mechanic can tell you anything he wants.
The BD member in high standing can't put out any falsehoods as his peers will make the feathers fly .. his feathers.
However just like here BD is endless posting of opinions laced w facts. But the opinions are of a much higher caliber and laced w (usually) others so your chances of getting a "bum steer" is very unlikely.
I would consider BD information WAY higher quality than any one mechanic. A mechanic may tell you all kinds of stuff that he wouldn't if other mechanics were listening.
The information in your owners manual likely would be the product of several to many engineers. Some of your questions may be questions like how to remove a fuel injector. That would be a good question for a mechanic. Some of your questions may be questions like what are te advantages of pre-chamber injection over direct injection or what are the advantages of synthetic oil? These are questions best suited for an engineer. How much power will it take to reach hull speed and what is the usable speed range of a specific boat? These are good questions for the naval architect.
The value of answers for questions depend largely on the knowledge and experience of the person asked. For mechanical marine engine and related mechanical questions a mechanic is an excellent source. But I've seen many mechanics give out information that is more engineering than mechanics.