> It implies that a ship "weighs" less if you put it in a huge vat of alcohol, as well.

Yep, since alcohol is less dense than water, a ship floating in alcohol will sit deeper in alcohol than in water. Water density varies with salinity and temperature, too, so they have different Plimsoll or "load lines" painted on ships to estimate their weight in tropical fresh water (TF), freshwater (F), tropical saltwater (TF), summer temperate seawater (S), winter temperate seawater (W) or Winter North Atlantic (WNA)...a ship of a given weight will sit much higher in WNA water than TF water...