I recommend having a leak company come out and tell you exactly what is leaking. I had one and was sure I knew what the problem was and was expecting a VERY expensive fix. Called the leak company before we proceeded with the expected fix "in case" - it was something totally different and the leak company even fixed it on the spot for us. To me it was worth the $300 to know exactly what was going on!

The skimmer is where to look. Fill the pool, turn off the pump and use liquid food coloring to dye test around the skimmer. Pay attention to grout joints and where the plastic meets the pool. You are looking for the dye to get drawn in (out of the pool). Seek & repair.
If its a raised spa it is likley equalizing to pool level due to a failed valve or check valve, not a direct cause of water loss.

i agree with the previous post. The spa is equalizing into the pool and separate from your leak. Sounds like something has gotten wedged into the check valve and won't let it close all the way. You definately need to have the leak pinpointed. Guessing can cuase you a lot of needless expense and it could be a couple of small leaks in different locations. If it stops at the skimmer that is probably the culprit. You can putty that with A& B pool putty. Won't last forever but is an inexpensive band-aid and does last for quite awwhile, as well as can be applied under water.