Why should you never use automotive electrical parts on a boat vessel?

Other answer:

the main reason is explosion hazard if there are gas fumes present. starters are sealed alternators have flame arrestor screens over cooling vents. as do distributers. bilge pumps blower fans are all sealed to prevent sparks. even the boat wiring may melt and stink but it will not support a flame. fuel hose in a bilge must be coast guard rated it takes several minutes to burn thru. giving you time to get out of the boat in case of fire. if you want to use automotive parts you are playing Russian roulette. why take the chance.
never say never – but salt water and air is a recipe for corrosion, particularly if there are dissimilar metals involved, so parts intended for dry land will rot out. E.g. "brass" is often brass-plated steel and will rust. Unanodised aluminium will corrode – I've seen a chainsaw motor reduced to powder and metal flakes by continued exposure to salt spray. Also in recent regimes there may be a requirement for electrical parts like alternators to be spark-proof if there is gasoline or propane/butane in use.
On the other hand my boat engine is based on a Ford car engine and takes standard spark plugs, coil, points etc. which have lasted for many years (manufactured before said regimes started, though I have changed the alternator)
Another thing to add to the good answers above: marine grade wiring is very different than automotive. It's tinned to prevent corrosion. And connections should ALWAYS be made with heat shrink tubing. Wire nuts are sure to cause problems over time.
More likely to cause an electrical spark in an enclosed engine compartment where gas fume may be present.
Ian K:
Umm… moisture 😉