How about maple syrup ? Would probably make a mess in your hair, in your ears, etc.

How about 10W40 motor oil?

Come on. Really. Muscle training is what properly designed weight machines are for. . . Without all the mess.

This would be more about endurance. They have cords which you tie to your feet, and you stay in one place.

i don't think that buying thousands of gallons of special liquid will be economical, no matter what you use, nevermind the fact that increased viscosity almost always means increased density, which means you will float higher in the water, decreasing displacement, and maintaining the same, or possibly decreasing resistance.

people need to be less skeptical on this website. the day before an idea is brilliant, it's usually a crazy idea.

pro swimmers need the best quality training money can buy. they spend money on expensive custom made drag suits. i think it would be very helpful to create a pull that somehow created more resistance.

good idea, let me know how it goes if you do pursue it

I think this would be counter-productive. The swimmers are going to race in water, not something denser or thicker. They would be best off training in the element that they will actually compete in. If you want to improve your skill in a specific activity, it will be most effective to practice that exact activity, not something else. This simple truth is overlooked so often, it's amazing.