This is where the sentence is:
But that
"I do not doubt but that my uncle will come." The sentence is shorter and more clear without the word but. "I have no idea but that the crew was drowned." Here but is necessary. Without it the opposite meaning would be conveyed.
Great.
So just like it says "I have no idea that the crew was drowned" = I don't know that the crew was drowned.
"I have no idea but that the crew was drowned" = I know only that the crew was drowned (I don't know, for example, what happened to the passengers, or what the ship was carrying, etc)