In nautical terms this stripe is known as a lubber-line (or plimsol line I believe). This is used to ascertain whether a vessel is misloaded by comparing the line with that of the sea-surface. - Al E Vator
If I may be allowed a little thread drift from the northern hemisphere, the Plimsoll line, or international load line, marks the maximum safe draught for a merchant vessel in various operating conditions (eg salt, or fresh water such as the Great Lakes), whilst the lubber's line is a fixed line marked on the glass of a ship's compass to indicate the direction in which direction the bows of the ship are pointing.
Meanwhile, back in WA .....
Jack