I tried setting a custom white balance on my GH5 today off the sandy bottom around 20 meters depth (60'). Camera appeared to do a CWB just fine, no error messages like you sometimes get with a Sony. I took a test shot and later opened it up in Lightroom. To my surprise, the CWB that the camera had set was 9200k/+149 tint.
This was obviously nowhere near warm enough given the conditions and the depth, so the results were pretty blue/green.
This got me thinking -- can the GH5 actually do a CWB warmer than 10000k, or is that a limit? When setting the white balance manually, you can't go warmer than 10000k, but I had assumed a CWB bypassed that limit. Has anyone successfully gotten a custom white balance warmer than that? If not, this has obvious implications for video, where you can't just shoot raw and set the white balance warmer after the fact in light room. It might also explain why I've never been able to get satisfactory ambient-light only results using custom white balance at depths greater than 10 meters.