Emerald Cruises occupies a specific and well-defined space in the travel market: the accessible premium tier below its parent brand Scenic, offering a contemporary luxury experience on European rivers and warm-water oceans without the ultra-luxury pricing or the expedition complexity.

SeaDream Yacht Club is the smallest ultra-luxury cruise line covered in this guide, and in many ways the most singular. Its two mega-yachts each carry a maximum of 112 guests served by a crew of 95, a near one-to-one ratio that produces a level of anticipatory...