The graduations on a plimsoll line are used to ensure a ship isn't overloaded with cargo. I'm not sure the correct nautical term for these graduations, but as the ship sits higher or lower in the water depending on the salinity/saltiness of the waters, I've called this entry salinity.
Oh, and the ship is the Cutty Sark, I went with the children on Friday and can highly recommend it, it's beautiful.
Nick