Generally alloys can selectively corrode, so with a combination of gold and iron the iron may still be attacked. But certainly some gold alloys provide improved hardness without being very vulnerable.

For a boat the gold coating would need to include all metal parts in contact with the water (rudder pins, prop shaft, etc) and it would be an interesting experiment to learn whether it really prevents fouling (I can imagine that wear and damage to the coating might provide cervices for growth to anchor)