You got it.
Those big cruise ships "create" all their drinking water through the use of massive watermakers.

They're really just really big reverse osmosis, super high pressure water filters. Sea water in, fresh water out.
Although, in the process of stripping the "sea" out of the water they also strip out all the minerals and other goodies that give drinking water its flavor.

When I was sailing across the Pacific, we had a watermaker on board (much smaller than the cruise ship variety, but same principle) and I always found the water it produced to be a bit flat and not so tasty. But one cant argue with its life supporting properties when supplies are limited.
"Water, water, everywhere, nor any drop to drink"
- The Rime of the Ancient Mariner by S.T. Coleridge