This bug was fixed in Patch 1.21.32
My Sims become 2-Dimensional and don’t slide over the slide at all. They do appear at the other end after the end of the animation.
This issue is caused by Intel Graphics Chips (Mac and PC) and Nvidia cards with old drivers.
If your computer has both a graphics chip and a graphics card, make sure the game runs on the card.
Also make sure that your drivers are up-to-date.
EA has released a beta fix: Beta Fix For Lawn Water Slide Animation Issue On Backyard Stuff Pack. Log into the forums and download the file, then place it in your mods folder. Please only use if you actually have the issue.