As someone who lives in Vermont, I can promise you that syrup doesn't exactly have the consistency of something you'd want to swim in, but scientists in 2004 decided to do it anyway. Researchers from the University of Minnesota filled a swimming pool with guar gum, ice cream, and shampoo to create a sticking substance that was twice as thick as water.
The reason for this was to see whether or not it was slower to swim in this syrupy mixture than it is to swim in water. To determine this they got a group of 16 competitive or recreational swimmers to swim through both and timed them.
Interestingly enough, one of the hardest things to do for this endeavor was to get approval for it. As one can imagine, it's hard to convince authorities that it'll be okay for this stuff to go down the drain.
But the important part is the results, which is that there was no real difference. Despite there being a difference between the viscosity between the two liquids, "neither water nor syrup produc[ed] consistently faster times." (1)
An explanation for why this can happen is that speed for humans isn't directly related to what you swim in, but rather what shape you are. Physics (and other sciency things that I don't fully understand) makes it that having a narrow frontal profile and powerful muscles will get you through nearly any liquid viscosity.
https://www.nature.com/articles/news040920-2