The Watchman of the title isn't a big blue guy who likes to walk around naked.
Unfortunately.
Jean Louise's watchman comes from a Biblical verse: Isaiah 21:6. In the book, the preacher, Mr. Stone says, "For thus hath the Lord said unto me / Go, set a watchman, let him declare what he seeth" (7.11).
Needing to make everything, including the Bible, about herself, Jean Louise later thinks that Mr. Stone should have given her a watchman: "I need a watchman to lead me around and declare what he seeth every hour on the hour" (13.135).
That sounds a little annoying, actually, like Siri telling you what to do every sixty minutes.
Finally, Uncle Jack tells Jean Louise that, "every man's watchman is his conscience. There is no such thing as a collective conscious" (18.89). In other words, think for yourself. The title is central to Jean Louise's personal quest—a quest to separate herself and think on her own. It's scary, and she has to set a strong watchman, i.e. follow her conscience, in order to do it.
To Birth a Mockingbird
Because of To Kill a Mockingbird, we have to be bird watchers in this book. Are there any mockingbirds, a symbol that would eventually come to the forefront when Harper Lee took this draft and developed her classic novel?
Yes, there are.
In one notable scene, Jean Louise gets up early and stands outside:
On any other day she would have stood barefoot on the wet grass listening to the mockingbirds' early service; she would have pondered over the meaninglessness of silent, austere beauty renewing itself with every sunrise and going ungazed at by half the world. (12.4)
Ignoring the fact that it wouldn't be silent if the mockingbirds were singing, this passage reveals some of the thought that went behind the symbolism in To Kill a Mockingbird.