4, fish can drown, the same way humans can asphyxiate. Some sharks can never stop moving as there isn't enough oxygen in the nearby water to facilitate their metabolism, so they need to continuously pump water through their gills, they stop swimming they keel over, literally.
I'm a graduated marine biologist with nits to pick.
Not my post. I'm a third person who came along already knowing this fact, and when you posted a link, I was expecting it to be this Scishow video which explains why there's no such thing as a fish (or, alternatively, there is such a thing as a fish, and people are included in that category) in much greater depth.
But yes, QI is great! I miss the days when whole episodes could easily be found on YouTube.
The TL;DW is: in cladistics (which is the preferred way of classifying animals these days), there's no way to include all things that we would call "fish" without also including all mammals, reptiles, and birds. So either people are fish, or fish don't exist.