I believe it is the line above which the sea should not rise. It's marked out the outside of the craft and if the line is below water the craft is too overloaded. I could be wrong, my sea-faring...
The ship you are looking for might be the Mary Rose, of the English Navy in the 16th Century. The Mary Rose may not have been called a ship-of-the-line, but for all intent and purpose that is what...