All I can really say about this special is that it was interesting seeing Takagi interact with another character besides Nishikata for once. The show's normal dynamic has them pretty much only ever spending time with each other, so having the double-date setup was a neat way to get some insights on how Takagi talks about Nishikata with other people, and what she's like when she isn't just bullying him.
Other than that, it's the swimsuit episode. An excuse for the artist to draw the female cast with more skin showing than normal. The titular water slide works as a good teasing mechanism but honestly I'm already starting to get a little sick of their whole dynamic and wish Nishikata would just grow a pair already. 7/10