The author is trying to prove that it is not more dangerous to swim at night, and their reasoning for this is that all shark attacks have occured during the day, and that swimmers are too scared to swim during the night.
This reasoning is flawed since the author admits that swimmers prefer to swim in the day, so there are simply fewer swimmers at night who could potentially be attacked by a shark. Thus, it shouldn't be a surprise that sharks attack swimmers during the day, since that is the time when swimmers are actually swimming.
The conclusion is not based on any source that we know of (although there must be some source of data about swimming patterns and when shark attacks occur), so we have no idea if the conclusion is drawn from reliable data or not. The author is not at all citing popular opinion as a source. The reference to popular opinion is just rhetorical framing for the author's opinion.