Why there are two miles
The statute mile is 5,280 feet, exactly 1,609.344 m. The nautical mile was built for navigation instead: it is one minute of arc of latitude, fixed internationally in 1929 at exactly 1,852 m, so a chart's latitude scale doubles as a distance ruler. That makes a nautical mile 1.150779… statute miles, about 15% longer. A knot is one nautical mile per hour, which is why aviation and shipping speeds convert to mph with the same factor — see the knots to mph converter.
miles = nautical miles × 1.150779448 | nautical miles = miles ÷ 1.150779448
Rough conversion in your head
Add 15% to nautical miles to get statute miles: 100 nmi → 115 mi. For kilometres, nautical miles times 1.85 is almost exact: 100 nmi → 185 km. The 12-nautical-mile territorial waters limit is 13.8 statute miles or 22.2 km.