Disclaimer: I have no experience of the IBIS in the GH5.
However, my former Sony MC50 video camera had an OIS that was able to lessen, but not completely eliminate, the slight swaying caused by fin action whilst shooting video and swimming at the same time. The Sony had couple of settings for the OIS and I shot test videos with various settings before I selected the setting that best matched my swimming style. I once forgot the recording on after a dive and got wonderful footage of the camera being handed over to the boat and then thrown under a bench. Thereafter the OIS kept compensating the rolling motion of the boat.
PS. you can also lessen the swaying motion by performing much smaller fin strokes, e.g. modified frog or flutter kick.
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