Don't Look Down
The lookout is the eyes of the ship. Usually stationed in the crows' nest, high above the deck, they must be vigilant in keeping watch. Early detection gives warships the edge in combat, and keeps merchant shipping out of danger. Being exposed to the elements and incoming fire makes the lookout vulnerable, and if the ship lurches suddenly, they may be facing a long drop.
Even with the advent of radar and other electronic detection methods, the lookout is still vitally important. The Imperial Japanese Navy taught the US Navy a hard lesson in visual detection during several battles early in World War II, and even today, radar can't pick up everything.