No wonder you have trouble with this - it's an idiom mixed with a metaphor. There is no set phrase "the sure-thing boat."
But your understanding is basically correct. If you are are afraid of the possibilities of failure and only want a "sure thing" (something that is certain to happen), then you won't take risks, the same way that a boat that is afraid of the ocean won't venture from from the dock.
Does that help?
[not a teacher]