water has one density, or it is no water :S
Besides changes related to pressure and temperature, I believe there's also a difference in density between sea water (average 1025 kg/m³ on the surface), and fresh water (average 1000 kg/m³).
That's way we(I was to become to be one... but... I had an small accident!) Engineers that design ships always use the density of the water in our calculation. A boat that was designed for going in sea-water, can in
worse case get unstable in fresh-water, some engineers do also have different calculation for the temperature of the water in there calculation since the temperature of the water can have some effects on the ship.
there is also people that believe that aerodynamic is not important, it is, specially on large ships. for wind can blow a ship very easy out of course. but an tall ship will have an effect like an sail just as an brake.
There are many factors an engineers have to think about. And it's only the best engineers that think about all those small factors, that truly design something create.