Everything is deadly when it floods, with water floods being responsible for about half of all deaths from global natural disasters [0].
The Wikipedia article you linked to describes the event but says nothing about swimming through it. There's a Scientific American article that analyzed this based on the Reynolds number [1] and arrived at a conclusion that you can't swim through molasses via regular symmetric motions and would need something different, which sounds quite appropriate for the analogy.
https://en.m.wikipedia.org/wiki/Great_Molasses_Flood