Seriously though... you could swap Gilligan--character and actor--for Dr. Smith and it wouldn't amount to a ant hill's difference in the two shows.
That's completely untrue. They were both screwups, but in very different ways. Gilligan was naive and clumsy but well-intentioned; he wanted to be helpful but just wasn't any good at it. And while he was easily frightened, he was able to overcome it when his friends needed him. Dr. Smith, by contrast, was self-absorbed, greedy, habitually dishonest, manipulative, and lazy, and his cowardice was so overwhelming that he'd willingly abandon the others to save his skin. The Gilligan's Island
character who most resembles Dr. Smith is Mr. Howell.