Why did Clark dream of the department store lady in National Lampoon's Christmas Vacation? No reason is given in the movie as to why he dreams of her in the pool. It seems unrelated to the overall plot of the movie.
There is a recurring theme in the Vacation movies of Clark fantasizing about other women despite having a rather attractive wife.
Additionally, this scene was almost certainly done as an homage to the famous pool scene with Phoebe Cates in Fast Times At Ridgemont High
Returning as the sole writer, John Hughes knew what worked in the first Vacation better than anyone else. That’s probably why there are more callbacks and repeated jokes from the first film than there were in European Vacation. The Griswolds’ car flies off road and lands haphazardly at their destination. Check. Clark flirts and fantasizes about another woman. Check. A family member’s pet gets accidentally killed by Clark. Check. Clark freaks out and has a curse-filled diatribe in front of his family. Check. Cousin Eddie. Check. In fact, with some of these bits, Hughes doubled down. Christmas Vacation might not be as R-rated as the first film, but Clark has even more freak-outs. His character is more manic and off-the-rails. Every character tick, every impulse, is emphasized for maximum Clark Griswoldness. Likewise not only is Cousin Eddie back, he has a larger role and is even more madcap. Thus, the two strongest characters are given the most focus and they’re given more opportunities to be as funny as possible.
So they just reused same recurring elements in the whole film series.