More than 70% of the earth's surface is covered by water, so it makes sense to see the world by sea. Lengthy and expensive, world cruises were once the domain of super-rich and well-heeled retirees with enough disposable income and time to travel. However, today's far-flung world cruises can often be booked in segments, making them more accessible to a wider range of travelers whose imaginations are only constrained by budget or time.