: On the subject of freshwater/saltwater, wouldn't this make a pretty big
: difference? Not only does the density change, but would there be
: differences in friction?

: Anybody have a PhD in the Molecular motion of H2O?

Not me, but I know this guy who knows this guy who's really smart, and he says...
The relative density (specific gravity) of fresh water (at 15 deg C) is 0.998 while salt water (35% salinity) is 1.026, making a hull float 5 to 6% higher in seawater.
Mass density for fresh water is 102, sea water 104.6. Mass density relates to resistance to motion. BTW, this resistance is 816 times greater in water than in air.
Viscosity of sea water is 0.000116 kg s/(m*m), fresh is 0.000122. Viscosity varies more with temperature than with salinity. The viscosity of water is some 65 to 70 times greater than that of air, depending on temp, pressure, salinity.
Skin friction is (mass density * velocity) * wetted area * coef of friction. The coeficient of friction is derived from the ratio of viscosity to mass density. Some interesting height numbers on surface roughness...

Rubbed down and polished: 0.0005mm

Smooth paint, rubbed down: 0.05mm

Bare wood: 0.5mm

Average barnacle: 5.0mm

The smart guy here would be Pierre Gutelle who wrote "The Design of Sailing Yachts" 1979 (translated from French 1983) by International Marine. Where Kinney (Skene's) and Marchaj pick and choose their subjects Gutelle makes a concerted effort to touch all the bases and back it up with empirical method. A really helpful ref if your into design.

So what's it mean? Like several people have already said- Not only are any gains/losses small but their affects are nulled by opposing gains/losses.
Smoothing your bottom is probly the only thing that will make a discernable difference in performance. That done, kick back with a Guiness or two and admire your image in the hull, or think about how many angels can fit on the head of a pin, or how many kayakers does it take to make a forum? Glub, glub glub...