A very risky thing to do unless you are competent at that sort of thing.
It's not illegal if you remain 500ft+ away from boats etc. I don't know if buoys count; he was certainly quite close to those. Taking a bit of a risk to show the tail number
My view is that putting this on Youtube is a foolish thing to do and merely brings GA into disrepute (as if that was needed, in some quarters). It's a bit like those FPV model plane flying videos; they are great fun but putting them on Youtube is just going to result in a widespread ban on model plane flying in "interesting" places, which is a great pity.