Like fine European hotels or the fanciest boutique properties, the ships of best luxury cruise lines are designed to appeal to people who seek — and can afford — an elegant style of travel. Champagne flows. You might snack on caviar. Exquisite cuisine is prepared a la minute and accompanied by pours of fine wines. Onboard lounges are wonderfully intimate, with no mass-market anything. When you rest your head, it’s on a luxury mattress topped with fine linens.