This really demonstrate how much these things are subjective. It's perfect to my eyes.
For my type of diving I never worked at less than ISO 1000 with my old GH2 and GH3. I know they are noisy but when you are there you have to bite the bullet.
I was curious to understand if there is a real improvement with the now obsolete GH4 and I could serenely work at ISO 1250.
Thank you for your example.
I think it depends on what size you're viewing the footage in -- and I suggest downloading the original rather than looking at the Vimeo stream, as the Vimeo compression may be smoothing out some of the noise. On a phone or laptop display, the noise might not be obvious, but once blown up to HDTV size, it's pretty obvious (to me anyway).
Also, add color correction to try to adjust the purple water column for example, and the noise becomes considerably worse. That can be counteracted to some extent by using noise reduction in post, of course.