Knock knock \ Who’s there? \ Dwayne!
Dwayne the bathtub, I’m dwowning.
As promised - here is an awful knock-knock joke.
Take a moment to bask in its glory - it is rare to find such a bad knock-knock joke, so this perhaps should be considered an achievement of sorts.
This joke relies on the listener not recognizing that the jokester is originally speaking with a lisp. Dwayne is a perfectly reasonable name, and who would answer "drain" with a lisp to the question "who's there?" That would be insanity - so this expectation on the listener to hear "Dwayne" where Dwayne was said is totally reasonable.
The punchline "dwayne the bathtub, I'm dwowning" makes the listener realize that the jokester is talking with a lisp, so they (the listener) misunderstood the first line.
Now - the dumbest part is that the jokester apparently wants the listener to buy into a world where someone is knocking on their door to tell them that the knocker wants the bathtub drained because they are drowning. This would only make sense if the bathtub were on one side of the door, and the door were reachable by a person that is drowning in the bathtub, and the mechanism to control the drain is on the other side of the door.
Nevermind, it is no less dumb than the plot to certain James Bond movies, actually.
I think that this next little bit will be a study of horrible knock-knock jokes, so if you can't stand that, come back in a little while.