[davidmurdoch]

To a lesser effect, there's also the surface tension of the water, which can be adjusted with salt and borates.

[HPsquared]

Density and viscosity must also play a role, or are those managed via temperature control?

[davidmurdoch]

Seems very plausible. I haven't noticed a speed difference in my own pool with temperature changes, but that might just be because the temperature itself can be so distracting. But I do feel like a salty pool makes me faster (maybe from buoyancy), and borates makes the water feel smoother and less "sticky" (something about surface tension changes with borates, it even makes the surface look different).

[HPsquared]

Oh and of course the cooling effect. That's a factor for long distances especially if the water is too warm.