Panel 1: That's because we got Liz actually interacting with Dee. Deanna tells her that she can make the current Miracle Dress sleeveless without too much work. Her only question is if Elly is going to mind. Liz assures her she won't. I smell impending doom here, don't you? Flapping and honking about a ruined dress and day is my forecast.
Panel 2: Deanna notes that Liz is actually taking the plunge and getting married. Liz's response: she'll allow Deanna the analogy.
Panel 3: In fact, she responds in kind by saying that the two of them are good swimmers. Dee answers back that Liz is just getting her feet wet...
Panel 4: ...but marriage ain't no backyard pool. Deanna, of course, wouldn't say that if she hadn't married a Patterson or Patterson-like life form.
Summary: Deanna is welcoming her sister-in-law into the world of professional martyrdom. Watching a spineless twinkie like her dish out advice when we all know she knows nothing about anything is wonderful, nasty fun. It's even funnier because she's got a great audience in Liz, the floating corpse.