Brooke Burns is used for both the pretty and ugly versions of Katrina, this is a prosthetic change.
Cadence (the burns victim girl in hospital) is played by Brianna Gardner, the daughter of special-effects makeup artist Tony Gardner who worked on the film.
At McIntosh's, the restaurant where Mauricio (Jason Alexander) calls Hal and makes him stop seeing women's inner beauty, when Hal sees Rosemary as she actually is, he says, "Hey! Now she's eating my clams casino!". In "Seinfeld" (1990), this is the same meal that George Costanza, played by Alexander, orders.
When Rosemary jumps in the pool, the huge splash was created by two wide hoses (which Gwyneth Paltrow had to dive between) which shot off the streams of water on either side of her, simulating a huge splash of water.