I have to admit it, . . .

. . . I'm a dunce. Will someone help me out?

I just do NOT get the joke told by Bumpy. He was talking about the incident that changed his attitude about the military. A young guy came to him and said, "Uncle, you like warm water?" Everyone in the scene seems to enjoy the story and laughs.

I've gone back and watched it to try to catch the meaning.
No success.

Anyone?