Is the attached what you were trying to achieve? It now has a top (water surface) that is a single horizontal plane and a base that is an inclined single plane.
Your file had two glaring problems. One was that the water surface wasn’t flat (try achieving that in the real world!). The second is that your model was a long way from the origin and there was something close to the origin. That probably accounts for the file size as you had an environment that was way bigger than it needed to be.
Here’s what I did.
First, I selected everything but excluded the pool group and hit delete. I then moved the pool group to the origin.
I took your model and got rid of everything except the outline of the water surface. I then created a simple rectangle much larger than the pool a little below the outline and extruded it to create a box 1600mm deep. I used drape under Sandbox tools to draw the outline onto my (now flat) surface. That created the level water surface.
I then extruded the pool shape down to the bottom plane. I now had two identical planes 1600mm apart with vertical faces around the pool.
I then chose to make the short wall at the shallow end consistently 900mm deep and chose an arbitrary point opposite for the full 1600mm deep end (as there is no wall parallel to it already). I drew a horizontal line at the shallow end 900mm below the surface. At each end of the enclosing box I joined the ends of this line to the line opposite representing the deep end. This created an inclined plane representing the bottom of the pool. I then selected this plane and did Intersect with model to create a new outline of the pool on the inclined surface. I then erased any sections of vertical wall below that plane and got rid of the enclosing box.
There may be a more elegant way of doing this (@DaveR ?) but it works.
My file is only 172KB.
Pool.skp (167.5 KB)