aamoroso,
How do you intend to do the cutting for real?
If you install the 2x4s first, then trim them in place, to a pool-like shape, cutting at the assembly level is absolutely valid in SolidWorks.
If you intend to cut the 2x4s to size ahead of time, then you should think about some DFMA procedures, such as minimizing the number of different 2x4s, and minimizing the number of weird shaped cuts.
Failing all that, you can use a design table to control the length of your boards. You will have to work out each length individually, and it will not be parametric with your assembly.
There are all sorts of ways to abuse Microsoft Excel. Consider using an external spreadsheet to define your shape. When you have something working, copy the data, values only, to your design table.
It seems to me that a rectangular swimming pool sitting in a rectangular area can be assembled using a very small number of different 2x4s. If you want a hyperbolic swimming pool sitting in a star shaped area, it is going to cost you.
JHG