Do most boats with mechanical gauges use a standard nylon tube or braided stainless?

xtandard nylon or copper tube.... braided would be outrageous & not needed. None of the pressures are high enough to require it. Not to worry.

Don't use nylon or copper for fuel pressure.

I agree with that- I put mechanical fuel pressure gauges directly mounted to the fuel lines at the carbs or efi fuel logs for setting accurate pressures and as a cross check of the electronic gauges on the dash. The only mechanical gauges really needed are boost & water pressure as the sensors don't like water exposure. I was referring to burst pressures as far as the nylon goes (oil pressure, water, boost or even fuel)...... Fatique could be a problem for copper, not so with the nylon.

Thanks guys. I was really only speaking of boost, vacuum, and water.