You can’t jet to Nevis, but you can swim there — if you first fly to St. Kitts, Nevis’ sister island. Most people take the 20-minute ferry from St. Kitts to Nevis, its tiny West Indies island neighbor, after jetting to Kitts on a commercial flight. But once a year, hundreds of swimmers jump into the Caribbean Sea in the annual Cross Channel Swim and cover the 2-1/2-mile- trip through “The Narrows” on their own steam. They leave their luggage behind, of course.