I think its kinda impossible to do this, but you can make good looking water (that also flows down the slide) with a blue part and beams with the right texture
As a lot of people said, if you have a slope it’s best to use custom water. There’s quite a lot of guidance on scripting buoyancy on the devforum, and since it is quite a small area it should be straightforward to do.
I think theirs a plugin to fix this, “part to terrain” The reason it does this is because rather than being a slope, each section going down is its own flat-floor section