[SamReidHughes]

Change the density and the swimmer's buoyancy will change. Change the viscosity and the optimal swimming algorithm will change. As you increase a liquid's viscosity, the swimmer loses its ability to coast on the inertia provided by previous effort. At a high enough viscosity (given a swimmer of constant size and strength), it's as if the swimmer has no inertia. The movement of the swimmer and fluid will be effectively a function of what forces the swimmer is putting on the fluid at that point in time, because any inertia will immediately get "stolen" by the fluid.

For example, scallops can swim in water, but with a high enough viscosity (or a microscopically sized scallop), their swimming style would get them nowhere. See http://en.wikipedia.org/wiki/Scallop_theorem . At high enough viscosities, the best way for a human to swim underwater would be to grow new skin on the scalp and reabsorb excess skin at the feet.

Viscous fluids are better at stealing the effort you've put in, so you're going to have a harder time swimming in them, unless you can find a good way to push yourself along without donating your velocity back to the fluid.