You may search for a vessel by selecting the flag (ensign) the vessel flies. Most are based on the traditional state flag, but with notable stylistic modifications. Flags are very important at sea, where they can mean the difference between life and death, and consequently the rules and regulations for the flying of flags are strictly enforced.
Ensigns are national maritime flags flown by ships, at the stern, from a gaff, or from the yard-arm. In some countries (e.g., the United Kingdom, Italy, Russia or Australia) there are distinct naval and merchant ensigns, while in other countries (e.g., United States or France) the merchant and naval ensigns are identical to the national flag flown on land. Ensigns are usually required to be flown when entering and leaving harbour, when sailing through foreign waters, and when the ship is signaled to do so by a warship.