I really love the comic XKCD, but his output today is just plain wrong.
In it, two people are trying to catch a swimming Michael Phelps with a net, but can't because he is too fast. So they decide to dump Jello in the pool to slow him down.
Unfortunately, this will do nothing to slow him down. While the viscosity of the water will increase and hence the drag, he will also be able to push off the viscous water more effectively. The net effect will be that nothing changes.
This has in fact been demonstrated back in 2004 by one of my previous chemical engineering professors here at the University of Minnesota, Ed Cussler. Himself an avid swimmer, he was somehow able to convince the maintenance people here to let him gel the pool with guar gum. Additionally, he was able to convince the swim team to swim in the goo. A comparison of times in water vs. goo showed no statistical difference, and in doing so, won an Ignobel Prize in 2005.
Back to the cartoon, the mouseover shows that the two pursuers were unsuccessful in catching Phelps anyway, as he ate all the Jello, a nod to his well known appetite. Right result, but wrong reason.
UPDATE: I have additional comments on this matter here.