But why was there water in the hydraulic system in the first place?
Likely contamination of ground handling equipment [1]. Unfortunately can happen. I wonder if the hydraulic fluid is hygroscopic or something?
1. https://www.pacaf.af.mil/Portals/6/documents/3_AIB%20Report....
But what ground handling equipment? Does the hydraulic fluid need to be replaced after normal usage? Any intervention is an opportunity for mishaps, failure and sabotage.
I'd expect the unlimited F-35 budget to come up with some formula that resists fire, cold, heat and pressure and doesn't need refills and/or with water detectors and other safety-critical chemical sensors in the hydraulic system.