There are plenty of ways to travel in luxury by sea. A cruise liner cares for every practical detail, but its scale means strict schedules, crowds and little room for unique experiences. A private yacht charter, by contrast, can feel thrillingly free and discreet, but usually comes with eye-watering costs, logistical difficulties and a relative lack of fancy facilities like spa treatments and on-board entertainment. The latest launches and expansions from respected names in hospitality occupy...