Watertight surfaces can't be unwrapped in one time without seams
The issue comes from the absence of any seam in the portion of mesh that not belong to the pool's stairs. As there is no edge marked as seam, you are asking the software to unwrap a shape that is "solid", that cannot be profitably unwrapped, an operation that leads to the "default" result (the same you get when choosing the reset function).
That is true even for simple object like the shown pyramid. You simply cannot unwrap watertight mesh (unless the are bi-dimensional): you need at least to make an hole somewhere or, preferrably, mark some seams:
Here's a possible result using the provided mesh:
Seams has been made all around the floor of the pool (on both sides) and the resulting ring has been cut in half at some point.