Z7. I was working in DX crop mode:
AF-Mode: AF-C and spot.
VR on the lens ( AF-S 70-200mm f/2.8E): Off
IBIS: Off
Custom Function a3 Focus Tracking with Lock-on setting: 3 (default)
Conclusion: nearly perfect.
Water polo is difficult because there are fast waving arms and splashing water coming crossing across the face of the player being focused on. In this situation No matter your focus technique you are bound to miss some frames with any camera (even the Sony A9)
I also used a fluid head (Benro S8) and tripod (ProMediaGear TR344L with levelling base) supporting the camera and lens. Panning and tilt drag on the head were both set to zero tension. This let me concentrate my focus and energy on watching the action and timing my shots.
I see some people complaining about EVF focus lag but that was true with SLRs and DSLRs as well. Back then I learned that if I saw the action in the viewfinder I probably missed the shot, so I see this as an issue unrelated to EVFs. The moral is: It pays to learn our equipment and to work on one's predictive timing technique.