There are those who are sticklers for grammatical rules of thumb, for rules that mustn’t be broken. I don’t know though. The sailor who says “as I am let to know that it is” is suggesting the mystery behind his line. It’s a way to say something about Hamlet without saying it. If he were trying to follow the writing rules, he’d say, “I’ve heard your name is Horatio” which would be more efficient but less interesting.