What effect would water that is twice as dense, while still liquid, have if a human were to try and swim in it?

Question

In my world there are two bodies of water. One acts as a radiation shield and is part of the atmosphere, so it needs some (undecided) force to keep it in place. The other is ideally twice as dense as what we consider regular water and I’m trying to figure out what effect the denser water would have on mobility while submerged.

Answer 1

Use the Dead Sea as a reference

I would suggest learning more about what it's like to swim in the Dead Sea. Thanks to its salt content, the water there is 25% more dense than fresh water. Even though that's not close to the 2x you wanted, you should be able to find plenty of research material. There is one thing I know:

while submerged

It's not going to be easy to be submerged. As you read about the Dead Sea, you'll see people talking about how it's almost impossible to swim in it due to how buoyant people are in it. In your 2x density liquid, that will be even more significant - with good balance you could probably make it look like you were walking through waist-deep fluid while floating.

In order to sink, you'll need to bring along weights that weigh a little bit more than you do (don't forget that the weights will have some buoyancy themselves). I think that the weights you need to stay submerged will actually affect your movement much more than the 2x dense liquid will.

Answer 2

More of a physics question than worldbuilding question really, but you would have twice the buoyancy with twice the pressure at the same depth as regular water, and twice the force/drag/lift. Water at double the density is 33% thicker than honey or molasses, but without the stickiness or viscosity if it still water. I couldn't actually find any liquid, common or uncommon, anywhere near twice as dense as water. There's a huge gap between something like honey and molasses which is ~1.5x as dense and bromine which is ~3x as dense, and then mercury which is much much more dense. And of all those the only one you would have experienced in daily life is honey or molasses.

All this means is you would have to also stroke twice as hard to swim in it, but you will also produce twice the force, but you also experience twice the drag. So your swim strokes would probably be slower. I think most people would interpret it as being easier to swim in.