Originally Posted by biffer
...until someone worked out that boats are heavier in fresh water, load a boat in sea water to the gunnels and when it enters fresh water, down she goes.
To be accurate, "less buoyant" rather than "heavier".
Sea water weighs more than an equal amount of fresh water (due to the dissolved salts), so a given weight ship will displace less sea water than fresh, thus riding higher in salt water.
Shows up in small boats as well, as a slight reduction in top speed when going from salt to fresh water.