The mantis shrimp starting from 5:22 have soft eyes. Try output to 4K, and may be when you only output to 1080p, Youtube compress it further. From my experience, if you upload 4K to Youtube, even play back at 1080p, the quality is still good.
Dust and particle in the water shouldn't affect sharpness if you shoot manual focus. What you want to do is turn the peaking on, move your camera so that the peaking is on the eye of the subject, so if particle float by, it won't affect your sharpness, diving in murky water (water with sand stir up) would be a different story.